[X86] Replace an APInt that is guaranteed to be 8-bits with just an 'unsigned'

We're already mixing this APInt with other 'unsigned' variables. This allows us to use regular comparison operators instead of needing to use APInt::ult or APInt::uge. And it removes a later conversion from APInt to unsigned.

I might be adding another combine to this function and this will probably simplify the logic required for that.

llvm-svn: 347684