[CostModel][X86] Improve f64/v2f64/v4f64 FMUL costs on AVX1 targets to account for slower btver2

BTVER2 has a weaker f64 multiplier that other AVX1-era targets, so we need to bump the worst case cost slightly - llvm-mca reports the new vectorization in simplebb is beneficial on btver2, bdver2 and sandybridge AVX1 targets