[InstCombine] canonicalize (bitcast (extractelement X)) --> (extractelement(bitcast X))

This change was discussed in D15392. It allows us to remove the fold that was added
in:
http://reviews.llvm.org/r255261

...and it will allow us to generalize this fold:
http://reviews.llvm.org/rL112232

while preserving the order of bitcast + extract that it produces and testing shows
is better handled by the backend.

Note that the existing check for "isVectorTy()" wasn't strong enough in general
and specifically because: x86_mmx. It's not a vector, but it's not vectorizable
either. So here we check VectorType::isValidElementType() directly before 
proceeding with the transform.

llvm-svn: 255433