[CostModel][X86] Improve masked load/store AVX1/AVX2 costs

[CostModel][X86] Improve masked load/store AVX1/AVX2 costs

A mixture of internal tests and review of the scheduler models indicates we're overestimating the cost of a masked load, which we're estimating at 4x regular memory ops - more realistic values indicates that its closer to 2x. Masked stores costs are a lot more diverse but 8x is roughly in the middle of the range.

e.g. SandyBridge
defm : X86WriteRes<WriteFMaskedLoad, [SBPort23,SBPort05], 8, [1,2], 3>;
defm : X86WriteRes<WriteFMaskedLoadY, [SBPort23,SBPort05], 9, [1,2], 3>;
defm : X86WriteRes<WriteFMaskedStore, [SBPort4,SBPort01,SBPort23], 5, [1,1,1], 3>;
defm : X86WriteRes<WriteFMaskedStoreY, [SBPort4,SBPort01,SBPort23], 5, [1,1,1], 3>;

e.g. Btver2
defm : X86WriteRes<WriteFMaskedLoad, [JLAGU, JFPU01, JFPX], 6, [1, 2, 2], 1>;
defm : X86WriteRes<WriteFMaskedLoadY, [JLAGU, JFPU01, JFPX], 6, [2, 4, 4], 2>;
defm : X86WriteRes<WriteFMaskedStore, [JSAGU, JFPU01, JFPX], 6, [1, 1, 4], 1>;
defm : X86WriteRes<WriteFMaskedStoreY, [JSAGU, JFPU01, JFPX], 6, [2, 2, 4], 2>;

Differential Revision: https://reviews.llvm.org/D61257

llvm-svn: 362338