[SVE] Fix invalid usages of getNumElements in ShuffleVectorInstruction

[ctetreau]

Summary

Fix invalid usages of getNumElements identified by test case
LLVM.Transforms/InstCombine::vscale_extractelement.ll.

changesLength: Since the length of the llvm::SmallVector shufflemask
is related to the minimum number of elements in a scalable vector, it is
fine to just get the Min field of the ElementCount

isIdentityWithExtract: Since it is not possible to express the mask
needed for this pattern for scalable vectors, we can just bail before
calling getNumElements()

[sdesmalen]

LGTM

llvm/include/llvm/IR/Instructions.h

public:
   ArrayRef<int> getShuffleMask() const { return ShuffleMask; }
 
   /// Return true if this shuffle returns a vector with a different number of
   /// elements than its source vectors.
   /// Examples: shufflevector <4 x n> A, <4 x n> B, <1,2,3>
   ///           shufflevector <4 x n> A, <4 x n> B, <1,2,3,4,5>
   bool changesLength() const {
     unsigned NumSourceElts =
-        cast<VectorType>(Op<0>()->getType())->getNumElements();
+        cast<VectorType>(Op<0>()->getType())->getElementCount().Min;
     unsigned NumMaskElts = ShuffleMask.size();
     return NumSourceElts != NumMaskElts;
   }
 
   /// Return true if this shuffle returns a vector with a greater number of
   /// elements than its source vectors.
   /// Example: shufflevector <2 x n> A, <2 x n> B, <1,2,3>
   bool increasesLength() const {

llvm/lib/IR/Instructions.cpp

if (Mask[i] != -1)
       return false;
 
   return true;
 }
 
 bool ShuffleVectorInst::isIdentityWithExtract() const {
   if (isa<UndefValue>(Op<2>()))
     return false;
-  int NumOpElts = cast<VectorType>(Op<0>()->getType())->getNumElements();
-  int NumMaskElts = getType()->getNumElements();
+
+  // FIXME: Not currently possible to express a shuffle mask for a scalable
+  // vector for this case
+  if (isa<ScalableVectorType>(getType()))
+    return false;
+
+  int NumOpElts = cast<FixedVectorType>(Op<0>()->getType())->getNumElements();
+  int NumMaskElts = cast<FixedVectorType>(getType())->getNumElements();
   if (NumMaskElts >= NumOpElts)
     return false;
 
   return isIdentityMaskImpl(getShuffleMask(), NumOpElts);
 }
 
 bool ShuffleVectorInst::isConcat() const {
   // Vector concatenation is differentiated from identity with padding.