[X86] selectLEAAddr - relax heuristic to only require one operand to be a...

[X86] selectLEAAddr - relax heuristic to only require one operand to be a MathWithFlags op (PR46809)

As suggested by @craig.topper, relaxing LEA matching to only require the ADD to be fed from a single op with EFLAGS helps avoid duplication when the EFLAGS are consumed in a later, dependent instruction.

There was some concern about whether the heuristic is too simple, not taking into account lost loads that can't fold by using a LEA, but some basic tests (included in select-lea.ll) don't suggest that's really a problem.

Differential Revision: https://reviews.llvm.org/D118128