[DAGCombiner][PowerPC] Remove isMulhCheaperThanMulShift TLI hook, Use...

[DAGCombiner][PowerPC] Remove isMulhCheaperThanMulShift TLI hook, Use isOperationLegalOrCustom directly instead.

MULH is often expanded on targets.
This patch removes the isMulhCheaperThanMulShift hook and uses
isOperationLegalOrCustom instead.

Differential Revision: https://reviews.llvm.org/D80485