[misc] Add is_path_all_dense to SNode

[k-ye]

Related issue = N/A

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[yuanming-hu]

Thanks! LGTM.

[codecov]

Codecov Report

Merging #1538 into master will increase coverage by 0.60%.
The diff coverage is 87.60%.

@@            Coverage Diff             @@
##           master    #1538      +/-   ##
==========================================
+ Coverage   86.10%   86.71%   +0.60%     
==========================================
  Files          19       19              
  Lines        3548     3680     +132     
  Branches      631      652      +21     
==========================================
+ Hits         3055     3191     +136     
+ Misses        365      355      -10     
- Partials      128      134       +6     

Impacted Files	Coverage Δ
python/taichi/lang/kernel.py	82.77% <ø> (+1.20%)	⬆️
python/taichi/lang/impl.py	89.41% <63.15%> (-1.14%)	⬇️
python/taichi/lang/matrix.py	91.55% <71.79%> (+0.12%)	⬆️
python/taichi/lang/meta.py	64.70% <75.00%> (+2.20%)	⬆️
python/taichi/lang/__init__.py	80.05% <78.94%> (+0.58%)	⬆️
python/taichi/lang/transformer.py	93.70% <95.00%> (+0.08%)	⬆️
python/taichi/lang/ops.py	93.33% <95.55%> (+0.20%)	⬆️
python/taichi/lang/snode.py	93.69% <100.00%> (+6.65%)	⬆️
python/taichi/lang/util.py	62.26% <100.00%> (-2.45%)	⬇️
... and 3 more

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