[X86] Remove the separate SMUL8/UMUL8 X86ISD opcodes by merging with...

[X86] Remove the separate SMUL8/UMUL8 X86ISD opcodes by merging with SMUL/UMUL. Remove the second result from X86ISD::UMUL.

All of these use custom isel so we can pretty easily detect the differences in the custom code in X86ISelDAGToDAG. The ISD opcodes just need to express the desired semantics not the details of how they would be selected by isel. So unifying them lets us remove the special casing from lowering.

llvm-svn: 350206