Fix wrong safemath lowering

[Y-Nak]

What was wrong?

1. Right shift operation isn't lowered sar when operands are signed type
2. negate operation isn't lowered into safe_sub call.

How was it fixed?

🔪

• OPTIONAL: Update Spec if applicable
• Add entry to the release notes (may forgo for trivial changes)
• Clean up commit history

[codecov-commenter]

Codecov Report

Merging #723 (52256eb) into master (f810961) will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #723      +/-   ##
==========================================
- Coverage   81.68%   81.65%   -0.03%     
==========================================
  Files         105      105              
  Lines       11843    11851       +8     
==========================================
+ Hits         9674     9677       +3     
- Misses       2169     2174       +5     

Impacted Files	Coverage Δ
crates/codegen/src/yul/isel/function.rs	80.50% <60.00%> (-0.56%)	⬇️
crates/mir/src/ir/value.rs	61.53% <100.00%> (+3.20%)	⬆️
crates/analyzer/src/context.rs	60.83% <0.00%> (-0.84%)	⬇️

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