[InstCombine] Skip GEP of bitcast transform with opaque pointers

This transform is fundamentally incompatible with opaque pointers.
Usually we would not hit it anyway because the bitcast is folded
away earlier, but due to worklist order it might survive until
here, so make sure we bail out explicitly.

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -2082,10 +2082,15 @@ Instruction *InstCombinerImpl::visitGEPOfGEP(GetElementPtrInst &GEP,
 // Note that we may have also stripped an address space cast in between.
 Instruction *InstCombinerImpl::visitGEPOfBitcast(BitCastInst *BCI,
                                                  GetElementPtrInst &GEP) {
+  // With opaque pointers, there is no pointer element type we can use to
+  // adjust the GEP type.
+  PointerType *SrcType = cast<PointerType>(BCI->getSrcTy());
+  if (SrcType->isOpaque())
+    return nullptr;
+
   Type *GEPEltType = GEP.getSourceElementType();
+  Type *SrcEltType = SrcType->getNonOpaquePointerElementType();
   Value *SrcOp = BCI->getOperand(0);
-  PointerType *SrcType = cast<PointerType>(BCI->getSrcTy());
-  Type *SrcEltType = SrcType->getPointerElementType();
 
   // GEP directly using the source operand if this GEP is accessing an element
   // of a bitcasted pointer to vector or array of the same dimensions: