fix a bug in computeConstantOffset

[bjjwwang]

for negative offset, we do not need to get FlattenIdx.

[codecov]

Codecov Report

Merging #1177 (9f67fe4) into master (3d18a81) will decrease coverage by 0.01%.
Report is 4 commits behind head on master.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1177      +/-   ##
==========================================
- Coverage   64.75%   64.74%   -0.01%     
==========================================
  Files         222      222              
  Lines       23481    23483       +2     
==========================================
  Hits        15204    15204              
- Misses       8277     8279       +2     

Files Changed	Coverage Δ
svf/lib/MemoryModel/AccessPath.cpp	4.54% <0.00%> (-0.15%)	⬇️

... and 2 files with indirect coverage changes