Teach VMCore to constant fold shufflevectors with constant operands.

Teach VMCore to constant fold shufflevectors with constant operands.
This allows us to compile:

#include <emmintrin.h>
typedef __m128i VSInt16;
typedef short vSInt16 __attribute__ ((__vector_size__ (16)));
VSInt16 t3() {
  return (VSInt16)((vSInt16)_mm_set1_epi16(6518));
}

into:

_t3:
	movaps	LCPI1_0, %xmm0
	ret

instead of:
_t3:
	movl	$6518, %eax
	movd	%eax, %xmm0
	pextrw	$0, %xmm0, %eax
	xorps	%xmm0, %xmm0
	pinsrw	$0, %eax, %xmm0
	punpcklwd	%xmm0, %xmm0
	pshufd	$0, %xmm0, %xmm0
	ret

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@44856 91177308-0d34-0410-b5e6-96231b3b80d8