make the pointee type related to gep more robust

[jumormt]

1. assert that we can have at most one pointer type (normally the first index that simply pieces the base pointer) when bridging a gep instruction.

2. remove the assertion checking in getelementNum because the pointee may be of a pointer type for the multi-level pointer, e.g., %2 = getelementptr inbounds i32*, i32** %1, ... the corresponding source code is like:

int getValue(int** arr, int x, int y) {
    return arr[x][y];
}

This can be transformed to: %2 = getelementptr inbounds [N x i32], [N x i32]* %1, ...
However, we do not know N without context information. int** implies non-contiguous blocks of memory. In this case, we conservatively return the max field limit.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (a6f6832) 64.51% compared to head (a11b255) 64.51%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1303   +/-   ##
=======================================
  Coverage   64.51%   64.51%           
=======================================
  Files         223      223           
  Lines       23833    23836    +3     
=======================================
+ Hits        15376    15378    +2     
- Misses       8457     8458    +1     

Files	Coverage Δ
svf-llvm/lib/SVFIRBuilder.cpp	78.37% <100.00%> (+0.08%)	⬆️
svf/lib/MemoryModel/AccessPath.cpp	3.15% <0.00%> (-0.04%)	⬇️