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
This was achieved using the 'cost-tables vs llvm-mca' script D103695
Also fix a missing pmullw v16i16 half-rate throughput as znver1 double-pumps - matches numbers from AMD SoG + Agner
