IRgen/ABI/x86_64: Avoid passing small structs using byval sometimes.

 - We do this when it is easy to determine that the backend will pass them on
   the stack properly by itself.

Currently LLVM codegen is really bad in some cases with byval, for example, on
the test case here (which is derived from Sema code, which likes to pass
SourceLocations around)::

  struct s47 { unsigned a; };
  void f47(int,int,int,int,int,int,struct s47);
  void test47(int a, struct s47 b) { f47(a, a, a, a, a, a, b); }

we used to emit code like this::

  ...
  movl	%esi, -8(%rbp)
  movl	-8(%rbp), %ecx
  movl	%ecx, (%rsp)
  ...

to handle moving the struct onto the stack, which is just appalling.

Now we generate::

  movl	%esi, (%rsp)

which seems better, no?

llvm-svn: 152462