[X86][SSE] Add support for combining PINSRW into a target shuffle.

[X86][SSE] Add support for combining PINSRW into a target shuffle.

Also add the ability to recognise PINSR(Vex, 0, Idx).

Targets shuffle combines won't replace multiple insertions with a bit mask until a depth of 3 or more, so we avoid codesize bloat.

The unnecessary vpblendw in clearupper8xi16a will be fixed in an upcoming patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293627 91177308-0d34-0410-b5e6-96231b3b80d8