[async] [opt] Field value killing analysis

[xumingkuan]

Related issue = #742

Changes:

• Analyze (with control flow graph) exactly which SNodes' original values before the OffloadedStmt are read inside the OffloadedStmt, and which GlobalPtrStmts' original value is still reachable at the end of the OffloadedStmt. These two cases, together with SNodes that are not element-wise written, should be the set of input value states in the TaskMeta.
• If the above analysis is accurate, RAW should be the only flow edge -- WAW becomes a dependency edge.
• Fix a bug that broke state-flow chains.

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[xumingkuan]

Not sure if the SFG is correct after this PR... I'll take a deeper look tmr.

[k-ye]

LGTM!

[codecov]

Codecov Report

Merging #1929 into master will increase coverage by 0.92%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1929      +/-   ##
==========================================
+ Coverage   42.80%   43.72%   +0.92%     
==========================================
  Files          45       45              
  Lines        6413     6207     -206     
  Branches     1101     1103       +2     
==========================================
- Hits         2745     2714      -31     
+ Misses       3498     3322     -176     
- Partials      170      171       +1     

Impacted Files	Coverage Δ
...aichi-dev/taichi/python/taichi/lang/ast_checker.py	70.58% <0.00%> (-1.64%)	⬇️
...s/build/taichi-dev/taichi/python/taichi/testing.py	75.00% <0.00%> (-0.72%)	⬇️
...ild/taichi-dev/taichi/python/taichi/lang/linalg.py	89.33% <0.00%> (-0.67%)	⬇️
...build/taichi-dev/taichi/python/taichi/lang/meta.py	64.38% <0.00%> (-0.49%)	⬇️
...d/taichi-dev/taichi/python/taichi/lang/__init__.py	40.45% <0.00%> (-0.33%)	⬇️
...build/taichi-dev/taichi/python/taichi/misc/util.py	18.27% <0.00%> (-0.24%)	⬇️
...build/taichi-dev/taichi/python/taichi/misc/task.py	0.00% <0.00%> (ø)
.../taichi-dev/taichi/python/taichi/tools/patterns.py	0.00% <0.00%> (ø)
.../build/taichi-dev/taichi/python/taichi/misc/gui.py	8.68% <0.00%> (+0.07%)	⬆️
...ild/taichi-dev/taichi/python/taichi/lang/kernel.py	71.39% <0.00%> (+0.13%)	⬆️
... and 9 more

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