[LAA] Add recursive IR walker for forked pointers

[LAA] Add recursive IR walker for forked pointers

This builds on the previous forked pointers patch, which only accepted
a single select as the pointer to check. A recursive function to walk
through IR has been added, which searches for either a loop-invariant
or addrec SCEV.

This will only handle a single fork at present, so selects of selects
or a GEP with a select for both the base and offset will be rejected.

There is also a recursion limit with a cli option to change it.

Reviewed By: fhahn, david-arm

Differential Revision: https://reviews.llvm.org/D108699