[X86][AVX] Fix lowering of X86ISD::VZEXT_MOVL for 128-bit -> 256-bit extension

The lowering patterns for X86ISD::VZEXT_MOVL for 128-bit to 256-bit vectors were just copying the lower xmm instead of actually masking off the first scalar using a blend.

Fix for PR25320.

Differential Revision: http://reviews.llvm.org/D14151

llvm-svn: 253561