GlobalISel: Handle selection of G_IMPLICIT_DEF in AArch64

A generic variant of IMPLICIT_DEF was added in r306875, but this
survives to selection and hits a `Cannot Select`. Add handling that
converts the note to a regular IMPLICIT_DEF.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307817 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/AArch64/AArch64InstructionSelector.cpp

@@ -1325,6 +1325,9 @@ bool AArch64InstructionSelector::select(MachineInstr &I) const {
   case TargetOpcode::G_VASTART:
     return STI.isTargetDarwin() ? selectVaStartDarwin(I, MF, MRI)
                                 : selectVaStartAAPCS(I, MF, MRI);
+  case TargetOpcode::G_IMPLICIT_DEF:
+    I.setDesc(TII.get(TargetOpcode::IMPLICIT_DEF));
+    return true;
   }
 
   return false;

test/CodeGen/AArch64/GlobalISel/select-implicit-def.mir

@@ -0,0 +1,30 @@
+# RUN: llc -O0 -mtriple=aarch64-- -run-pass=instruction-select -verify-machineinstrs -global-isel %s -o - | FileCheck %s
+
+--- |
+  target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"
+
+  define void @implicit_def() { ret void }
+...
+
+---
+# CHECK-LABEL: name: implicit_def
+name:            implicit_def
+legalized:       true
+regBankSelected: true
+# CHECK:      registers:
+# CHECK-NEXT:  - { id: 0, class: gpr32, preferred-register: '' }
+# CHECK-NEXT:  - { id: 1, class: gpr32, preferred-register: '' }
+registers:
+  - { id: 0, class: gpr }
+  - { id: 1, class: gpr }
+
+# CHECK:  body:
+# CHECK:    [[DEF:%[0-9]+]] = IMPLICIT_DEF
+# CHECK:    [[ADD:%[0-9]+]] = ADDWrr [[DEF]], [[DEF]]
+# CHECK:    %w0 = COPY [[ADD]]
+body:             |
+  bb.0:
+    %0(s32) = G_IMPLICIT_DEF
+    %1(s32) = G_ADD %0, %0
+    %w0 = COPY %1(s32)
+...