[X86] Convert to Constant arguments to MMX shift by i32 intrinsics to...

[X86] Convert to Constant arguments to MMX shift by i32 intrinsics to TargetConstant during lowering.

This allows us to use timm in the isel table which is more
consistent with other intrinsics that take an immediate now.

We can't declare the intrinsic as taking an ImmArg because we
need to match non-constants to the shift by MMX register
instruction which we do by mutating the intrinsic id during
lowering.

llvm-svn: 372537