X86/GlobalISel: Use generic version of splitToValueTypes

The custom insert of an unmerge and the callback weirdness should be
unnecessary. Since handleAssignments should now use
getRegisterTypeForCalling conv as SelectionDAG builder would, this
should now just be able to use the generic code. X86-32 relies on the
generated CCAssignFns not seeing illegal types and sharing code with
x86_64, so i64 values would incorrectly be assigned to 64-bit
registers.