[ARM] Fix phi operand order issue in MVEGatherScatterLowering (#78208)

With commuted operands on the phi node, the two old incoming values
could be removed in the wrong order, removing newly added operand
instead of the old one.

Author: davemgreen

llvm/lib/Target/ARM/MVEGatherScatterLowering.cpp

@@ -898,8 +898,8 @@ void MVEGatherScatterLowering::pushOutAdd(PHINode *&Phi,
   Phi->addIncoming(NewIndex, Phi->getIncomingBlock(StartIndex));
   Phi->addIncoming(Phi->getIncomingValue(IncrementIndex),
                    Phi->getIncomingBlock(IncrementIndex));
-  Phi->removeIncomingValue(IncrementIndex);
-  Phi->removeIncomingValue(StartIndex);
+  Phi->removeIncomingValue(1);
+  Phi->removeIncomingValue((unsigned)0);
 }
 
 void MVEGatherScatterLowering::pushOutMulShl(unsigned Opcode, PHINode *&Phi,

llvm/test/CodeGen/Thumb2/mve-gather-optimisation-deep.ll

@@ -62,11 +62,11 @@ end:
   ret void;
 }
 
-define arm_aapcs_vfpcc void @push_out_add_sub_block_c(i32* noalias nocapture readonly %data, i32* noalias nocapture %dst, i32 %n.vec) {
-; CHECK-LABEL: @push_out_add_sub_block_c(
+define arm_aapcs_vfpcc void @push_out_add_sub_block_commutedphi(i32* noalias nocapture readonly %data, i32* noalias nocapture %dst, i32 %n.vec) {
+; CHECK-LABEL: @push_out_add_sub_block_commutedphi(
 ; CHECK-NEXT:  vector.ph:
 ; CHECK-NEXT:    [[PUSHEDOUTADD:%.*]] = add <4 x i32> <i32 0, i32 2, i32 4, i32 6>, <i32 6, i32 6, i32 6, i32 6>
-; CHECK-NEXT:    [[SCALEDINDEX:%.*]] = shl <4 x i32> <i32 0, i32 2, i32 4, i32 6>, <i32 2, i32 2, i32 2, i32 2>
+; CHECK-NEXT:    [[SCALEDINDEX:%.*]] = shl <4 x i32> [[PUSHEDOUTADD]], <i32 2, i32 2, i32 2, i32 2>
 ; CHECK-NEXT:    [[TMP0:%.*]] = ptrtoint ptr [[DATA:%.*]] to i32
 ; CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <4 x i32> poison, i32 [[TMP0]], i64 0
 ; CHECK-NEXT:    [[DOTSPLAT:%.*]] = shufflevector <4 x i32> [[DOTSPLATINSERT]], <4 x i32> poison, <4 x i32> zeroinitializer