Only care about imm8 if the VF is fixed-width

Author: SamTebbs33

llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp

@@ -5344,10 +5344,10 @@ InstructionCost AArch64TTIImpl::getPartialReductionCost(
   } else
     return Invalid;
 
-  // AArch64 supports lowering mixed extensions to a usdot but only if the
-  // i8mm feature is available.
+  // AArch64 supports lowering mixed fixed-width extensions to a usdot but only
+  // if the i8mm feature is available.
   if (OpAExtend == TTI::PR_None || OpBExtend == TTI::PR_None ||
-      (OpAExtend != OpBExtend && !ST->hasMatMulInt8()))
+      (OpAExtend != OpBExtend && VF.isFixed() && !ST->hasMatMulInt8()))
     return Invalid;
 
   if (!BinOp || *BinOp != Instruction::Mul)