GlobalISel: Fix defined register of invariant.start (llvm#125664)

In contrast to SelectionDAG, GlobalISel created a new virtual register
for the return value of invariant.start, leaving subsequent users of the
invariant.start value with an undefined reference.
A minimal example:
```
  %tmp = alloca i32, align 4, addrspace(5)
  %tmpI = call ptr @llvm.invariant.start.p5(i64 4, ptr addrspace(5) %tmp) rust-lang#3
  call void @llvm.invariant.end.p5(ptr %tmpI, i64 4, ptr addrspace(5) %tmp) rust-lang#3
  store i32 %i, ptr %tmpI, align 4
```
Although the return value of invariant.start might not be intended for
any use beyond invariant.end (the fuzzer might not have created a
sensible situation here), an implicit definition of the corresponding
virtual register avoids a segfault in the target instruction selector
later.

This LLVM defect was identified via the AMD Fuzzing project.

llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp

@@ -2441,9 +2441,7 @@ bool IRTranslator::translateKnownIntrinsic(const CallInst &CI, Intrinsic::ID ID,
     return true;
   }
   case Intrinsic::invariant_start: {
-    LLT PtrTy = getLLTForType(*CI.getArgOperand(0)->getType(), *DL);
-    Register Undef = MRI->createGenericVirtualRegister(PtrTy);
-    MIRBuilder.buildUndef(Undef);
+    MIRBuilder.buildUndef(getOrCreateVReg(CI));
     return true;
   }
   case Intrinsic::invariant_end:

llvm/test/CodeGen/AArch64/GlobalISel/arm64-irtranslator.ll

@@ -2262,7 +2262,7 @@ declare ptr @llvm.invariant.start.p0(i64, ptr nocapture) readonly nounwind
 declare void @llvm.invariant.end.p0(ptr, i64, ptr nocapture) nounwind
 define void @test_invariant_intrin() {
 ; CHECK-LABEL: name: test_invariant_intrin
-; CHECK: %{{[0-9]+}}:_(s64) = G_IMPLICIT_DEF
+; CHECK: %{{[0-9]+}}:_(p0) = G_IMPLICIT_DEF
 ; CHECK-NEXT: RET_ReallyLR
   %x = alloca %t
   %inv = call ptr @llvm.invariant.start.p0(i64 8, ptr %x)

llvm/test/CodeGen/AMDGPU/GlobalISel/promote-dependency-on-invariant-result.ll

@@ -0,0 +1,23 @@
+; NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
+; RUN: llc -mtriple=amdgcn -mcpu=gfx90a -O0 -global-isel=true --stop-after=irtranslator -o - %s | FileCheck %s
+
+declare ptr @llvm.invariant.start.p5(i64 immarg, ptr addrspace(5) nocapture)
+declare void @llvm.invariant.end.p5(ptr, i64 immarg, ptr addrspace(5) nocapture)
+
+define void @use_invariant_promotable_lds(ptr addrspace(5) %arg, i32 %i) {
+  ; CHECK-LABEL: name: use_invariant_promotable_lds
+  ; CHECK: bb.1.bb:
+  ; CHECK-NEXT:   liveins: $vgpr0, $vgpr1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:_(p5) = COPY $vgpr0
+  ; CHECK-NEXT:   [[COPY1:%[0-9]+]]:_(s32) = COPY $vgpr1
+  ; CHECK-NEXT:   [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 0
+  ; CHECK-NEXT:   [[DEF:%[0-9]+]]:_(p0) = G_IMPLICIT_DEF
+  ; CHECK-NEXT:   G_STORE [[C]](s32), [[DEF]](p0) :: (store (s32) into %ir.tmp)
+  ; CHECK-NEXT:   SI_RETURN
+bb:
+  %tmp = call ptr @llvm.invariant.start.p5(i64 4, ptr addrspace(5) %arg)
+  call void @llvm.invariant.end.p5(ptr %tmp, i64 4, ptr addrspace(5) %arg)
+  store i32 0, ptr %tmp, align 4
+  ret void
+}