[X86] Allow LowerSELECT and LowerBRCOND to directly lower i8 UMULO/SMULO.

These require a different X86ISD node to be created than i16/i32/i64. I guess no one wanted to add the special code for that except in LowerXALUO. But now LowerXALUO, LowerSELECT, and LowerBRCOND all use a common helper function so they all share the special code.

Unfortunately, there are no test changes because we seem to correct the miss in a DAG combine later. I did verify it manually using test cases from xmulo.ll

llvm-svn: 350205