[X86] Use a specific list of MVTs in combineShiftRightArithmetic instead of...

[X86] Use a specific list of MVTs in combineShiftRightArithmetic instead of iterating over every integer VT and checking their size.

Previously, we were checking for MVTs with sizes betwen 8 and 64 which only includes i8, i16, i32, and i64 today. But I don't think we should assume that and should list the types that are legal for x86. I also don't think we need i64 since type legalization is guaranteed to split those up.

llvm-svn: 321058