Do not treat llvm.fake.use as a debug instruction (#128684)

The llvm.fake.use intrinsic is used to prevent certain values from being
optimized out for the benefit of debug info; it is not, however, a debug
or pseudo instruction itself and necessarily must not be treated as one,
since its purpose is to act like a normal instruction. In the original
commit that added them, the IR intrinsic however was treated as one in
`getPrevNonDebugInstruction` (but _not_ in `getNextNonDebugInstruction`,
or in the MIR equivalents). This patch correctly treats it as a
non-debug instruction.

Author: SLTozer

llvm/lib/IR/Instruction.cpp

@@ -1233,10 +1233,7 @@ Instruction::getNextNonDebugInstruction(bool SkipPseudoOp) const {
 const Instruction *
 Instruction::getPrevNonDebugInstruction(bool SkipPseudoOp) const {
   for (const Instruction *I = getPrevNode(); I; I = I->getPrevNode())
-    if (!isa<DbgInfoIntrinsic>(I) &&
-        !(SkipPseudoOp && isa<PseudoProbeInst>(I)) &&
-        !(isa<IntrinsicInst>(I) &&
-          cast<IntrinsicInst>(I)->getIntrinsicID() == Intrinsic::fake_use))
+    if (!isa<DbgInfoIntrinsic>(I) && !(SkipPseudoOp && isa<PseudoProbeInst>(I)))
       return I;
   return nullptr;
 }

llvm/test/Transforms/SimplifyCFG/X86/fake-use-considered-when-sinking.ll

@@ -0,0 +1,67 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -p="simplifycfg<sink-common-insts>" -S < %s | FileCheck %s
+
+;; Verify that fake uses are not ignored when sinking instructions in
+;; SimplifyCFG; when a fake use appears in only one incoming block they prevent
+;; further sinking, and when identical fake uses appear on both sides they
+;; are sunk normally.
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @foo(i1 %bool, ptr %p) {
+; CHECK-LABEL: define void @foo(
+; CHECK-SAME: i1 [[BOOL:%.*]], ptr [[P:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br i1 [[BOOL]], label %[[IF_ELSE:.*]], label %[[IF_THEN:.*]]
+; CHECK:       [[COMMON_RET:.*]]:
+; CHECK-NEXT:    ret void
+; CHECK:       [[IF_THEN]]:
+; CHECK-NEXT:    store ptr [[P]], ptr [[P]], align 8
+; CHECK-NEXT:    br label %[[COMMON_RET]]
+; CHECK:       [[IF_ELSE]]:
+; CHECK-NEXT:    store ptr [[P]], ptr [[P]], align 8
+; CHECK-NEXT:    notail call void (...) @llvm.fake.use(ptr [[P]])
+; CHECK-NEXT:    br label %[[COMMON_RET]]
+;
+entry:
+  br i1 %bool, label %if.else, label %if.then
+
+common.ret:                                       ; preds = %if.else, %if.then
+  ret void
+
+if.then:                                          ; preds = %entry
+  store ptr %p, ptr %p, align 8
+  br label %common.ret
+
+if.else:                                          ; preds = %entry
+  store ptr %p, ptr %p, align 8
+  notail call void (...) @llvm.fake.use(ptr %p)
+  br label %common.ret
+}
+
+define void @bar(i1 %bool, ptr %p) {
+; CHECK-LABEL: define void @bar(
+; CHECK-SAME: i1 [[BOOL:%.*]], ptr [[P:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    store ptr [[P]], ptr [[P]], align 8
+; CHECK-NEXT:    notail call void (...) @llvm.fake.use(ptr [[P]])
+; CHECK-NEXT:    ret void
+;
+entry:
+  br i1 %bool, label %if.else, label %if.then
+
+common.ret:                                       ; preds = %if.else, %if.then
+  ret void
+
+if.then:                                          ; preds = %entry
+  store ptr %p, ptr %p, align 8
+  notail call void (...) @llvm.fake.use(ptr %p)
+  br label %common.ret
+
+if.else:                                          ; preds = %entry
+  store ptr %p, ptr %p, align 8
+  notail call void (...) @llvm.fake.use(ptr %p)
+  br label %common.ret
+}
+