[X86] Add support for using 512-bit PSUBUS to combineSelect.

The code already support 128 and 256 and even knows to split 256 for AVX1. So we really just needed to stop looking for specific VTs and subtarget features and just look for legal VTs with i8/i16 elements.

While there, add some curly braces around outer if statement bodies that contain only another if. It makes all the closing curly braces look more regular.

llvm-svn: 340128