[AArch64][GlobalISel] Add ptradd_immed_chain combine to post-legalizer combiner.

Author: aemerson

llvm/lib/Target/AArch64/AArch64Combine.td

@@ -212,6 +212,7 @@ def AArch64PostLegalizerCombinerHelper
                         form_bitfield_extract, rotate_out_of_range,
                         icmp_to_true_false_known_bits, merge_unmerge,
                         select_combines, fold_merge_to_zext,
-                        constant_fold, identity_combines]> {
+                        constant_fold, identity_combines,
+                        ptr_add_immed_chain]> {
   let DisableRuleOption = "aarch64postlegalizercombiner-disable-rule";
 }

llvm/test/CodeGen/AArch64/GlobalISel/postlegalizer-combine-ptr-add-chain.mir

@@ -0,0 +1,28 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple aarch64-apple-ios  -run-pass=aarch64-postlegalizer-combiner --aarch64postlegalizercombinerhelper-only-enable-rule="ptr_add_immed_chain"  %s -o - -verify-machineinstrs | FileCheck %s
+# REQUIRES: asserts
+
+# Check that we fold two adds of constant offsets with G_PTR_ADD into a single G_PTR_ADD.
+---
+name: ptradd_chain
+tracksRegLiveness: true
+legalized: true
+body:             |
+  bb.1:
+    liveins: $x0
+
+    ; CHECK-LABEL: name: ptradd_chain
+    ; CHECK: liveins: $x0
+    ; CHECK: [[COPY:%[0-9]+]]:_(p0) = COPY $x0
+    ; CHECK: [[C:%[0-9]+]]:_(s64) = G_CONSTANT i64 16
+    ; CHECK: [[PTR_ADD:%[0-9]+]]:_(p0) = G_PTR_ADD [[COPY]], [[C]](s64)
+    ; CHECK: $x0 = COPY [[PTR_ADD]](p0)
+    ; CHECK: RET_ReallyLR implicit $x0
+    %0:_(p0) = COPY $x0
+    %1:_(s64) = G_CONSTANT i64 4
+    %2:_(s64) = G_CONSTANT i64 12
+    %3:_(p0) = G_PTR_ADD %0(p0), %1
+    %4:_(p0) = G_PTR_ADD %3(p0), %2
+    $x0 = COPY %4(p0)
+    RET_ReallyLR implicit $x0
+...