[CostModel][X86] Improve i8 and vXi8 MUL costs

We were treating vXi8 multiply as the sum of a trunc(mul(extend(),extend())) which diverged from the costs from llvm-mcaonce we extended beyond legal types

Use a modified version of the D103695 script to determine more accurate throughput/latency/codesize/size-latency cost estimates

Helps address some of the regressions identified in D148806