[DAGCombiner] Allow visitEXTRACT_SUBVECTOR to combine with BUILD_VECTORS between...

[DAGCombiner] Allow visitEXTRACT_SUBVECTOR to combine with BUILD_VECTORS between LegalizeVectorOps and LegalizeDAG.

BUILD_VECTORs aren't themselves legalized until LegalizeDAG so we should still be able to create an "illegal" one before that. This helps combine with BUILD_VECTORS that are introduced during LegalizeVectorOps due to unrolling.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327446 91177308-0d34-0410-b5e6-96231b3b80d8