[ARM] Fix Changed status in MVEGatherScatterLoweringPass.

Now that we are calling SimplifyInstructionsInBlock, make sure we update
Changed when it reports alterations.

Author: davemgreen

Diff for: llvm/lib/Target/ARM/MVEGatherScatterLowering.cpp

@@ -1166,7 +1166,7 @@ bool MVEGatherScatterLowering::runOnFunction(Function &F) {
   bool Changed = false;
 
   for (BasicBlock &BB : F) {
-    SimplifyInstructionsInBlock(&BB);
+    Changed |= SimplifyInstructionsInBlock(&BB);
 
     for (Instruction &I : BB) {
       IntrinsicInst *II = dyn_cast<IntrinsicInst>(&I);