[EarlyIfConversion] Avoid producing selects with identical operands

This extends the early-ifcvt pass to avoid a few more cases where the resulting
select instructions would have matching operands.  Additionally, we now use TII
to determine "sameness" of the operands so that as TII gets smarter, so too
will ifcvt.

The attached test case was bugpoint-reduced down from CINT2000/252.eon in the
test-suite. See: https://clang.godbolt.org/z/WvnrcrGEn

Differential Revision: https://reviews.llvm.org/D101508