Addresses PR#32791 (https://bugs.llvm.org//show_bug.cgi?id=32791).
When attempting to simplify an 'or' instruction, check whether its operands are
the results of 'select' instructions, and whether those instructions are the
inverse of one another (that is, their predicate are the inverse of one another,
but their true and false conditions are identical). If they are, the 'or' can
be simplified to a single 'select' instruction.