[X86] Add patterns for using movss/movsd for atomic load/store of f32/64....

[X86] Add patterns for using movss/movsd for atomic load/store of f32/64. Remove atomic fadd pseudos use isel patterns instead.

This patch adds patterns for turning bitcasted atomic load/store into movss/sd.

It also removes the pseudo instructions for atomic RMW fadd. Instead just adding isel patterns for folding an atomic load into addss/sd. And relying on the new movss/sd store pattern to handle the write part.

This also makes the fadd patterns use VEX and EVEX instructions when AVX or AVX512F are enabled.

Differential Revision: https://reviews.llvm.org/D60394

llvm-svn: 358215