[InstCombiner] Remove trivially dead llvm.allow.{runtime,ubsan}.check() (#84851)

Intrinsic declared to have sideeffects, but it's done only to prevent
moving it. Removing unused ones is OK.

Exacted from #84850 for easier review.

Author: vitalybuka

llvm/lib/Transforms/Utils/Local.cpp

@@ -475,6 +475,12 @@ bool llvm::wouldInstructionBeTriviallyDead(const Instruction *I,
         II->getIntrinsicID() == Intrinsic::launder_invariant_group)
       return true;
 
+    // Intrinsics declare sideeffects to prevent them from moving, but they are
+    // nops without users.
+    if (II->getIntrinsicID() == Intrinsic::allow_runtime_check ||
+        II->getIntrinsicID() == Intrinsic::allow_ubsan_check)
+      return true;
+
     if (II->isLifetimeStartOrEnd()) {
       auto *Arg = II->getArgOperand(1);
       // Lifetime intrinsics are dead when their right-hand is undef.

llvm/test/Transforms/InstCombine/allow-checks.ll

@@ -0,0 +1,44 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -passes=instcombine -S | FileCheck %s --implicit-check-not="call i1 @llvm.allow"
+
+define i1 @test_runtime()  {
+; CHECK-LABEL: @test_runtime(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[HOT:%.*]] = call i1 @llvm.allow.runtime.check(metadata !"test")
+; CHECK-NEXT:    ret i1 [[HOT]]
+;
+entry:
+  %allow = call i1 @llvm.allow.runtime.check(metadata !"test")
+  ret i1 %allow
+}
+
+define void @test_runtime_void()  {
+; CHECK-LABEL: @test_runtime_void(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    ret void
+;
+entry:
+  %allow = call i1 @llvm.allow.runtime.check(metadata !"test")
+  ret void
+}
+
+define i1 @test_ubsan()  {
+; CHECK-LABEL: @test_ubsan(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[HOT:%.*]] = call i1 @llvm.allow.ubsan.check(i8 11)
+; CHECK-NEXT:    ret i1 [[HOT]]
+;
+entry:
+  %allow = call i1 @llvm.allow.ubsan.check(i8 11)
+  ret i1 %allow
+}
+
+define void @test_ubsan_void()  {
+; CHECK-LABEL: @test_ubsan_void(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    ret void
+;
+entry:
+  %allow = call i1 @llvm.allow.ubsan.check(i8 11)
+  ret void
+}