[AArch64] Fold CSEL x, x, cc -> x

This can come up in rare situations, where a csel is created with
identical operands. These can be folded simply to the original value,
allowing the csel to be removed and further simplification to happen.

This patch also removes FCSEL as it is unused, not being produced
anywhere or lowered to anything.

Differential Revision: https://reviews.llvm.org/D101687