[X86][ArgumentPromotion] Allow Argument Promotion if caller and callee disagree on...

[X86][ArgumentPromotion] Allow Argument Promotion if caller and callee disagree on 512-bit vectors support if the arguments are scalar.

If one of caller/callee has disabled ZMM registers due to
prefer-vector-width=256, we were previously
disabling argument promotion as the ABI might be incompatible since
one side will split 512-bit vectors in this case.

But if we can see that the types are all scalar this shouldn't be
a problem.

This patch assumes that pointer element type reflects the type that
the argument will be promoted to.

Differential Revision: https://reviews.llvm.org/D78770