[X86] Passing union type through register

For example:

  union M256 {
    double d;
    __m256 m;
  };
  extern void foo1(union M256 A);
  union M256 m1;
  void test() {
    foo1(m1);
  }

clang will pass m1 through stack which does not follow the ABI.

Differential Revision: https://reviews.llvm.org/D78699