[X86] combineMulToPMADDWD - don't bitcast the source ops before splitting to...

[X86] combineMulToPMADDWD - don't bitcast the source ops before splitting to ensure we split the build vectors early

Fixes a regression on D127115 - splitting was creating extract_subvector(bitcast(build_vector())) patterns which prevented the build vectors being split before being bitcast to vXi16 types, resulting in various issues with further folding of the (now legal) build vectors