[Opt] Improve value_diff for better aliasing analysis

[xumingkuan]

Related issue = #1441 #1558 #656

(WIP)

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[codecov]

Codecov Report

Merging #1627 into master will increase coverage by 2.30%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1627      +/-   ##
==========================================
+ Coverage   63.22%   65.52%   +2.30%     
==========================================
  Files          19       19              
  Lines        3717     3542     -175     
  Branches      659      659              
==========================================
- Hits         2350     2321      -29     
+ Misses       1239     1092     -147     
- Partials      128      129       +1     

Impacted Files	Coverage Δ
python/taichi/lang/ast_checker.py	70.58% <0.00%> (-1.64%)	⬇️
python/taichi/lang/linalg.py	89.33% <0.00%> (-0.67%)	⬇️
python/taichi/lang/meta.py	64.17% <0.00%> (-0.53%)	⬇️
python/taichi/lang/__init__.py	54.76% <0.00%> (-0.45%)	⬇️
python/taichi/lang/shell.py	3.40% <0.00%> (+0.07%)	⬆️
python/taichi/lang/util.py	29.29% <0.00%> (+0.36%)	⬆️
python/taichi/lang/kernel.py	69.37% <0.00%> (+0.65%)	⬆️
python/taichi/lang/transformer.py	81.61% <0.00%> (+0.68%)	⬆️
python/taichi/lang/impl.py	68.26% <0.00%> (+2.17%)	⬆️
python/taichi/lang/snode.py	70.47% <0.00%> (+2.90%)	⬆️
... and 3 more

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