[X86] Remove dead code from X86ISelDAGToDAG.cpp multiply handling

Summary:
Lowering never creates X86ISD::UMUL for 8-bit types. X86ISD::UMUL8 is used instead. If X86ISD::UMUL 8-bit were ever used it would crash.

DAGCombiner replaces UMUL_LOHI/SMUL_LOHI with a wider MUL and a shift if the type twice as wide is legal. So we should never see i8 UMUL_LOHI/SMUL_LOHI. In fact I think there was a bug in part of the i8 code. Similar is true for i16 though without the bug.

Reviewers: RKSimon, spatel, zvi

Reviewed By: zvi

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38276

llvm-svn: 314430