If we're passing an i128 value and we no longer have enough argument
registers (only r9 unallocated), the value gets passed via the stack.
However, r9 is still allocated as a shadow register, which means that a
following i64 argument will not use it. This doesn't match the x86-64
psABI.
Fix this by making i128 arguments as requiring consecutive registers,
and then adding a custom CC lowering that will allocate both parts of
the i128 at the same time, either to register or to stack, without
reserving a shadow register.
Fixes https://github.com/llvm/llvm-project/issues/123935.
2068b1ba03