[AArch64] Avoid materializing 0.0 when generating FP SELECT

Transform `a == 0.0 ? 0.0 : x` to `a == 0.0 ? a : x` and `a != 0.0 ? x : 0.0`
to `a != 0.0 ? x : a` to avoid materializing 0.0 for FCSEL, since it does not
have to be materialized beforehand for FCMP, as it has a form that has 0.0
as an implicit operand.

Differential Revision: https://reviews.llvm.org/D24808

llvm-svn: 284531