When building a VVECTOR_SHUFFLE node from extract_element operations, make

When building a VVECTOR_SHUFFLE node from extract_element operations, make
sure to build it as SHUFFLE(X, undef, mask), not SHUFFLE(X, X, mask).

The later is not canonical form, and prevents the PPC splat pattern from
matching.  For a particular splat, we go from generating this:

	li r10, lo16(LCPI1_0)
	lis r11, ha16(LCPI1_0)
	lvx v3, r11, r10
	vperm v3, v2, v2, v3

to generating:

	vspltw v3, v2, 3

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@27236 91177308-0d34-0410-b5e6-96231b3b80d8