[InstCombine] Simplify pointer difference subtractions (GEP-GEP) where GEPs...

[InstCombine] Simplify pointer difference subtractions (GEP-GEP) where GEPs have other uses and one non-constant index

Summary:
Pointer difference simplifications currently happen only if input GEPs don't have other uses or their indexes are all constants, to avoid duplicating indexing arithmetic.

This patch enables cases with exactly one non-constant index among input GEPs to happen where there is no duplicated arithmetic or code size increase even if input GEPs have other uses.

For example, this patch allows "(&A[42][i]-&A[42][0])" --> "i", which didn't happen previously, if the input GEP(s) have other uses.

Reviewers: sanjoy, bkramer

Reviewed By: sanjoy

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D35499

llvm-svn: 309304