Make sure to use the result of the pattern to infer the result type of the

instruction, and the result type of the instruction to refine the pattern.
This allows us to write things like this:

def : Pat<(v2i64 (bitconvert (v16i8 VR128:$src))), (v2i64 VR128:$src)>;

as:
def : Pat<(v2i64 (bitconvert (v16i8 VR128:$src))), (VR128:$src)>

and fixes a ppc64 issue.

llvm-svn: 28863