[AIX] Extend int arguments to register width when passed in stack memory.

This is a follow up to the previous patch: [AIX] Implement caller
arguments passed in stack memory.

This corrects a defect in AIX 64-bit where an i32 is written to the
stack with stw (4 bytes) rather than the expected std (8 bytes.) Integer
arguments pass on the stack as images of their register representation.

I also took the opportunity to tidy up some of the calling convention
AIX tests I added in my last commit. This patch adds the missed assembly
expected output for the stack arg int case, which would have caught this
problem.

Differential Revision: https://reviews.llvm.org/D75126