DAG: Allow creating extract_vector_elt post-legalize

Fixes some combine issues for AMDGPU where we weren't
getting the many extract_vector_elt combines expected
in a future patch.

This should really be checking isOperationLegalOrCustom on
the extract. That improves a number of x86 lit tests, but
a few get stuck in an infinite loop from one place
where a similar looking extract is created. I have a
different workaround in the backend for that which
keeps many of those improvements, but also adds a few
regressions.

llvm-svn: 312730

Author: arsenm

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -13963,7 +13963,10 @@ SDValue DAGCombiner::visitEXTRACT_VECTOR_ELT(SDNode *N) {
     // FIXME: We should handle recursing on other vector shuffles and
     // scalar_to_vector here as well.
 
-    if (!LegalOperations) {
+    if (!LegalOperations ||
+        // FIXME: Should really be just isOperationLegalOrCustom.
+        TLI.isOperationLegal(ISD::EXTRACT_VECTOR_ELT, VT) ||
+        TLI.isOperationExpand(ISD::VECTOR_SHUFFLE, VT)) {
       EVT IndexTy = TLI.getVectorIdxTy(DAG.getDataLayout());
       return DAG.getNode(ISD::EXTRACT_VECTOR_ELT, SDLoc(N), NVT, SVInVec,
                          DAG.getConstant(OrigElt, SDLoc(SVOp), IndexTy));