Cleans up the code which had special cased some 32-bit optimization
which is unnecessary now that both 8-bit and 16-bit are also optimized.
When FEX does a VExtractToGPR, the result is zero extended to the full
GPR register size. This means we don't need to do a zero extend when
storing to a guest GPR.
Makes pextr{b,w} optimal now.
Needs #3088 merged first.
Ryan Houdek
33a2fbb896