[InstCombine] Canonicalize GEP of GEP by swapping constant-indexed GEP to the back

[InstCombine] Canonicalize GEP of GEP by swapping constant-indexed GEP to the back

Canonicalize GEP of GEP by swapping GEP with some suffix constant indices to the back (and GEP with all constant indices to the back of that), this allows more constant index GEP merging to happen. Exceptions are: If swapping violates use-def relations, or anti-optimizes LICM

For constant indexed GEP of GEP, if they cannot be merged directly, they will be casted to i8* and merged.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D125845