[FastISel][X86] Fix smul.with.overflow.i8 lowering.

Add custom lowering code for signed multiply instruction selection, because the
default FastISel instruction selection for ISD::MUL will use unsigned multiply
for the i8 type and signed multiply for all other types. This would set the
incorrect flags for the overflow check.

This fixes <rdar://problem/17549300>

llvm-svn: 212493