[X86, AVX] use blends instead of insert128 with index 0

[X86, AVX] use blends instead of insert128 with index 0

Another case of x86-specific shuffle strength reduction:
avoid generating insert*128 instructions with index 0 because
they are slower than their non-lane-changing blend equivalents.

Shuffle lowering already catches most of these cases, but
the zero vector case and some other paths such as in the
modified test in vector-shuffle-256-v32.ll were getting
through.

Differential Revision: http://reviews.llvm.org/D8366

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232773 91177308-0d34-0410-b5e6-96231b3b80d8