PoC: assume code ends with STOP

[chfast]

Assumption that code always ends with valid STOP instruction eliminates the "end-of-code" checks in the interpreter loop and in push data load. See individual commits for more information.

Benchmarks with evmone/Baseline interpreter build with Clang 11 on Haswell CPU 4.0 GHz, "main" benchmark set:

• time reduced by -2% to -11%,
• geometric mean: -7.2%.

Comparing o/baseline to o/valid_stop                                                                                                                                                                                              
Benchmark                                                                Time             CPU      Time Old      Time New       CPU Old       CPU New                                                                             
-----------------------------------------------------------------------------------------------------------------------------------------------------
baseline/execute/main/blake2b_huff/empty_mean                         -0.0617         -0.0618            25            23            25            23
baseline/execute/main/blake2b_huff/2805nulls_mean                     -0.1125         -0.1125           408           362           408           362
baseline/execute/main/blake2b_huff/5610nulls_mean                     -0.1004         -0.1004           783           704           783           704
baseline/execute/main/blake2b_huff/8415nulls_mean                     -0.1051         -0.1051          1148          1027          1148          1027
baseline/execute/main/blake2b_huff/65536nulls_mean                    -0.1039         -0.1039          8843          7924          8843          7924
baseline/execute/main/blake2b_shifts/2805nulls_mean                   -0.0740         -0.0740          3945          3653          3945          3653
baseline/execute/main/blake2b_shifts/5610nulls_mean                   -0.0810         -0.0810          8156          7495          8156          7495
baseline/execute/main/blake2b_shifts/8415nulls_mean                   -0.0814         -0.0814         12305         11303         12305         11303
baseline/execute/main/blake2b_shifts/65536nulls_mean                  -0.0897         -0.0897         89394         81377         89394         81378
baseline/execute/main/sha1_divs/empty_mean                            -0.0539         -0.0538            65            61            65            61
baseline/execute/main/sha1_divs/1351_mean                             -0.0570         -0.0570          1329          1254          1329          1254
baseline/execute/main/sha1_divs/2737_mean                             -0.0539         -0.0539          2593          2453          2593          2453
baseline/execute/main/sha1_divs/5311_mean                             -0.0573         -0.0573          5071          4780          5071          4780
baseline/execute/main/sha1_divs/65536_mean                            -0.0562         -0.0561         61711         58245         61711         58246
baseline/execute/main/sha1_shifts/empty_mean                          -0.0960         -0.0960            42            38            42            38
baseline/execute/main/sha1_shifts/1351_mean                           -0.1126         -0.1126           868           770           868           770
baseline/execute/main/sha1_shifts/2737_mean                           -0.1036         -0.1036          1677          1503          1677          1503
baseline/execute/main/sha1_shifts/5311_mean                           -0.1062         -0.1062          3287          2938          3287          2938
baseline/execute/main/sha1_shifts/65536_mean                          -0.0990         -0.0990         39656         35729         39656         35730
baseline/execute/main/weierstrudel/0_mean                             -0.0261         -0.0261           198           192           198           192
baseline/execute/main/weierstrudel/1_mean                             -0.0250         -0.0250           434           423           434           423
baseline/execute/main/weierstrudel/3_mean                             -0.0259         -0.0259           675           658           675           658
baseline/execute/main/weierstrudel/9_mean                             -0.0201         -0.0201          1393          1365          1393          1365
baseline/execute/main/weierstrudel/14_mean                            -0.0196         -0.0196          1992          1953          1992          1953

[chfast]

This optimization is enabled in #315 by copying and padding the original code.