[X86][CodeGenPrepare] Try to reuse IV's incremented value instead of adding the offse...

[X86][CodeGenPrepare] Try to reuse IV's incremented value instead of adding the offset, part 2

This patch enables the case where we do not completely eliminate offset.
Supposedly in this case we reduce live range overlap that never harms, but
since there are doubts this is true, this goes as a separate change.

Differential Revision: https://reviews.llvm.org/D96399
Reviewed By: reames