Use ConstantInt::getSigned instead of ConstantInt::get when creating a
negative alignment mask in EmitVAArgFromMemory. This is the same fix as
commit 8546294db95d (PR #176115) which addressed the issue in
EmitVAArgForHexagonLinux.
Added a test case that exercises the EmitVAArgFromMemory alignment path
using a struct that is both >8 bytes (to trigger EmitVAArgFromMemory)
and has 8-byte alignment (to trigger the alignment masking code).
99c9e5ebd6