Change EXTRACT_SUBVECTOR to require a constant index.

Change EXTRACT_SUBVECTOR to require a constant index.
We were never generating any of these nodes with variable indices, and there
was one legalizer function asserting on a non-constant index.  If we ever have
a need to support variable indices, we can add this back again.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122993 91177308-0d34-0410-b5e6-96231b3b80d8