[X86][SSE] Reduce FADD/FSUB/FMUL costs on later targets (PR36280)

Agner's tables indicate that for SSE42+ targets (Core2 and later) we can reduce the FADD/FSUB/FMUL costs down to 1, which should fix the Himeno benchmark.

Note: the AVX512 FDIV costs look rather dodgy, but this isn't part of this patch.

Differential Revision: https://reviews.llvm.org/D43733

llvm-svn: 326133