[Opt] Identical store/load elimination by control-flow graph

[xumingkuan]

Related issue = #656 #1248 #1324

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[codecov]

Codecov Report

Merging #1741 into master will increase coverage by 0.98%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1741      +/-   ##
==========================================
+ Coverage   43.45%   44.44%   +0.98%     
==========================================
  Files          44       44              
  Lines        6015     5810     -205     
  Branches     1042     1042              
==========================================
- Hits         2614     2582      -32     
+ Misses       3250     3078     -172     
+ Partials      151      150       -1     

Impacted Files	Coverage Δ
python/taichi/lang/ast_checker.py	70.58% <0.00%> (-1.64%)	⬇️
python/taichi/testing.py	77.41% <0.00%> (-0.71%)	⬇️
python/taichi/lang/linalg.py	89.33% <0.00%> (-0.67%)	⬇️
python/taichi/lang/meta.py	62.31% <0.00%> (-0.54%)	⬇️
python/taichi/lang/__init__.py	53.14% <0.00%> (-0.47%)	⬇️
python/taichi/misc/util.py	15.72% <0.00%> (-0.33%)	⬇️
python/taichi/misc/gui.py	0.00% <0.00%> (ø)
python/taichi/misc/task.py	0.00% <0.00%> (ø)
python/taichi/lang/shell.py	0.00% <0.00%> (ø)
python/taichi/tools/patterns.py	0.00% <0.00%> (ø)
... and 9 more

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[yuanming-hu]

Thank you! LGTM.