[AArch64] Remove redundant f{min,max}nm intrinsics.

The patch extends AArch64TTIImpl::instCombineIntrinsic to simplify
llvm.aarch64.neon.f{min,max}nm(a, a) -> a.

This helps with simplifying code written using the ACLE, e.g.
see https://godbolt.org/z/jYxsoc89c

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D125234