[AArch64][GlobalISel] Don't perform the mul const combine with G_PTR_ADD

[paquette]

Summary

A G_MUL + G_PTR_ADD can also be folded into a madd. So, conservatively, we shouldn't combine when the G_MUL is used by a G_PTR_ADD either.

llvm/lib/Target/AArch64/GISel/AArch64PostLegalizerCombiner.cpp

if (TrailingZeroes) {
     // folded into smul or umul.
     if (MRI.hasOneNonDBGUse(LHS) &&
         (isSignExtended(LHS, MRI) || isZeroExtended(LHS, MRI)))
       return false;
     // Conservatively do not lower to shift+add+shift if the mul might be
     // folded into madd or msub.
     if (MRI.hasOneNonDBGUse(Dst)) {
       MachineInstr &UseMI = *MRI.use_instr_begin(Dst);
-      if (UseMI.getOpcode() == TargetOpcode::G_ADD ||
-          UseMI.getOpcode() == TargetOpcode::G_SUB)
+      unsigned UseOpc = UseMI.getOpcode();
+      if (UseOpc == TargetOpcode::G_ADD || UseOpc == TargetOpcode::G_PTR_ADD ||
+          UseOpc == TargetOpcode::G_SUB)
         return false;
     }
   }
   // Use ShiftedConstValue instead of ConstValue to support both shift+add/sub
   // and shift+add+shift.
   APInt ShiftedConstValue = ConstValue.ashr(TrailingZeroes);
 
   unsigned ShiftAmt, AddSubOpc;

llvm/test/CodeGen/AArch64/GlobalISel/postlegalizercombiner-mulpow2.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple aarch64 -run-pass=aarch64-postlegalizer-combiner -verify-machineinstrs %s -o - | FileCheck %s
+
+...
+---
+name:            dont_combine_ptr_add
+legalized:       true
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $x0, $x1, $x2
+
+    ; G_PTR_ADD can become a madd, so we don't want to perform the combine.
+
+    ; CHECK-LABEL: name: dont_combine_ptr_add
+    ; CHECK: liveins: $x0, $x1, $x2
+    ; CHECK: %reg0:_(s64) = COPY $x0
+    ; CHECK: %ptr:_(p0) = COPY $x2
+    ; CHECK: %cst:_(s64) = G_CONSTANT i64 6
+    ; CHECK: %mul:_(s64) = nsw G_MUL %reg0, %cst
+    ; CHECK: %ptr_add:_(p0) = G_PTR_ADD %ptr, %mul(s64)
+    ; CHECK: $x0 = COPY %ptr_add(p0)
+    ; CHECK: RET_ReallyLR implicit $x0
+    %reg0:_(s64) = COPY $x0
+    %reg1:_(s64) = COPY $x1
+    %ptr:_(p0) = COPY $x2
+    %cst:_(s64) = G_CONSTANT i64 6
+    %mul:_(s64) = nsw G_MUL %reg0, %cst
+    %ptr_add:_(p0) = G_PTR_ADD %ptr, %mul
+    $x0 = COPY %ptr_add(p0)
+    RET_ReallyLR implicit $x0