[x86] try harder to form LEA from ADD to avoid flag conflicts (PR40483)

LEA doesn't affect flags, so use it more liberally to replace an ADD when
we know that the ADD operands affect flags.

In the motivating example from PR40483:
https://bugs.llvm.org/show_bug.cgi?id=40483
...this lets us avoid duplicating a math op just to avoid flag conflict.

As mentioned in the TODO comments, this heuristic can be extended to
fire more often if that leads to more improvements.

Differential Revision: https://reviews.llvm.org/D64707

llvm-svn: 366431