[Opt] Improve aliasing analysis for SNodes with trailing bits

[xumingkuan]

Related issue = #1558 #656
Replaces #1627
Benchmark(#stmts): unchanged.

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[codecov]

Codecov Report

Merging #1657 into master will decrease coverage by 0.19%.
The diff coverage is 32.43%.

@@            Coverage Diff             @@
##           master    #1657      +/-   ##
==========================================
- Coverage   44.40%   44.21%   -0.20%     
==========================================
  Files          41       41              
  Lines        5875     5892      +17     
  Branches     1022     1023       +1     
==========================================
- Hits         2609     2605       -4     
- Misses       3117     3138      +21     
  Partials      149      149              

Impacted Files	Coverage Δ
python/taichi/lang/meta.py	62.85% <0.00%> (-1.85%)	⬇️
python/taichi/misc/gui.py	0.00% <0.00%> (ø)
python/taichi/lang/snode.py	66.08% <25.00%> (-1.49%)	⬇️
python/taichi/lang/__init__.py	53.76% <50.00%> (-0.99%)	⬇️
python/taichi/lang/matrix.py	67.52% <50.00%> (+0.04%)	⬆️
python/taichi/lang/expr.py	63.42% <80.00%> (-0.37%)	⬇️
python/taichi/lang/impl.py	65.80% <100.00%> (ø)
python/taichi/lang/ops.py	43.62% <100.00%> (ø)
python/taichi/main.py	23.73% <0.00%> (ø)
... and 3 more

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[yuanming-hu]

Thanks! Looks good to me in general. Just a few minor issues. Please merge after they are resolved.