[X86] Fix i128 argument passing under SysV ABI

The x86_64 SysV ABI specifies that __int128 is passed either in
two registers (if available) or in a 16 byte aligned stack slot.
GCC implements this behavior. However, if only one free register
is available, LLVM will instead pass one half of the i128 in a
register, and the other on the stack.

Make sure that either both are passed in registers or both on the
stack.

Fixes https://github.com/llvm/llvm-project/issues/41784.
The patch is basically what craig.topper proposed to do there.

Differential Revision: https://reviews.llvm.org/D158169