X86: Fold EXTRACT_SUBVECTORs of a BUILD_VECTOR into a smaller BUILD_VECTOR.

That can usually be lowered efficiently and is common in sandybridge code.
It would be nice to do this in DAGCombiner but we can't insert arbitrary
BUILD_VECTORs this late.

Fixes PR15462.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176634 91177308-0d34-0410-b5e6-96231b3b80d8