[SelectionDAG][AArch64][X86] Move legalization of vector MULHS/MULHU from...

[SelectionDAG][AArch64][X86] Move legalization of vector MULHS/MULHU from LegalizeDAG to LegalizeVectorOps

I believe we should be legalizing these with the rest of vector binary operations. If any custom lowering is required for these nodes, this will give the DAG combine between LegalizeVectorOps and LegalizeDAG to run on the custom code before constant build_vectors are lowered in LegalizeDAG.

I've moved MULHU/MULHS handling in AArch64 from Lowering to isel. Moving the lowering earlier caused build_vector+extract_subvector simplifications to kick in which made the generated code worse.

Differential Revision: https://reviews.llvm.org/D54276

llvm-svn: 347902