[CodeGen] Fix up warnings in visitEXTRACT_SUBVECTOR

[david-arm]

Summary

It's perfectly valid to do certain DAG combines where we extract
subvectors from a concat vector when we have scalable vector types.
However, we can do this in a way that avoids generating compiler
warnings by replacing calls to getVectorNumElements() with
getVectorMinNumElements(). Due to the way subvector extracts are
designed to work with scalable vector types this is ok.

This eliminates some warnings from existing tests in this file:

llvm/test/CodeGen/AArch64/sve-intrinsics-loads.ll

[david-arm]

Please note that I tried my best to find tests that exposed the change in behaviour where I added the fixed length vector check, but to no avail! I put that extra check in anyway as I think the optimisation makes no sense for scalable vector types. All we're doing here is removing an infrequently used (I looked for fixed length vector types hitting this path and there weren't many) optimisation for scalable vector types, so I think so long as no existing tests break it should be ok.

[kmclaughlin]

LGTM!

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

if ((DestNumElts % SrcNumElts) == 0) {
             return DAG.getBitcast(NVT, NewExtract);
           }
         }
       }
     }
   }
 
   if (V.getOpcode() == ISD::CONCAT_VECTORS) {
-    unsigned ExtNumElts = NVT.getVectorNumElements();
+    unsigned ExtNumElts = NVT.getVectorMinNumElements();
     EVT ConcatSrcVT = V.getOperand(0).getValueType();
     assert(ConcatSrcVT.getVectorElementType() == NVT.getVectorElementType() &&
            "Concat and extract subvector do not change element type");
     assert((ExtIdx % ExtNumElts) == 0 &&
            "Extract index is not a multiple of the input vector length.");
 
-    unsigned ConcatSrcNumElts = ConcatSrcVT.getVectorNumElements();
+    unsigned ConcatSrcNumElts = ConcatSrcVT.getVectorMinNumElements();
     unsigned ConcatOpIdx = ExtIdx / ConcatSrcNumElts;
 
     // If the concatenated source types match this extract, it's a direct
     // simplification:
     // extract_subvec (concat V1, V2, ...), i --> Vi
     if (ConcatSrcNumElts == ExtNumElts)
       return V.getOperand(ConcatOpIdx);
 
     // If the concatenated source vectors are a multiple length of this extract,
     // then extract a fraction of one of those source vectors directly from a
     // concat operand. Example:
     //   v2i8 extract_subvec (v16i8 concat (v8i8 X), (v8i8 Y), 14 -->
     //   v2i8 extract_subvec v8i8 Y, 6
-    if (ConcatSrcNumElts % ExtNumElts == 0) {
+    if (NVT.isFixedLengthVector() && ConcatSrcNumElts % ExtNumElts == 0) {
       SDLoc DL(N);
       unsigned NewExtIdx = ExtIdx - ConcatOpIdx * ConcatSrcNumElts;
       assert(NewExtIdx + ExtNumElts <= ConcatSrcNumElts &&
              "Trying to extract from >1 concat operand?");
       assert(NewExtIdx % ExtNumElts == 0 &&
              "Extract index is not a multiple of the input vector length.");
       SDValue NewIndexC = DAG.getVectorIdxConstant(NewExtIdx, DL);
       return DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, NVT,