[LICM] Avoid assertion failure on stale MemoryDef

It can happen that the call is originally created as a MemoryDef,
and then later transforms show it is actually read-only and could
be a MemoryUse -- however, this is not guaranteed to be reflected
in MSSA.

Author: nikic

llvm/lib/Transforms/Scalar/LICM.cpp

@@ -1241,11 +1241,16 @@ bool llvm::canSinkOrHoistInst(Instruction &I, AAResults *AA, DominatorTree *DT,
     if (Behavior.doesNotAccessMemory())
       return true;
     if (Behavior.onlyReadsMemory()) {
+      // Might have stale MemoryDef for call that was later inferred to be
+      // read-only.
+      auto *MU = dyn_cast<MemoryUse>(MSSA->getMemoryAccess(CI));
+      if (!MU)
+        return false;
+
       // If we can prove there are no writes to the memory read by the call, we
       // can hoist or sink.
       return !pointerInvalidatedByLoop(
-          MSSA, cast<MemoryUse>(MSSA->getMemoryAccess(CI)), CurLoop, I, Flags,
-          /*InvariantGroup=*/false);
+          MSSA, MU, CurLoop, I, Flags, /*InvariantGroup=*/false);
     }
 
     if (Behavior.onlyWritesMemory()) {

llvm/test/Transforms/LICM/hoist-readonly-memory-def.ll

@@ -0,0 +1,39 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -passes="function(print<memoryssa>),cgscc(function-attrs),function(loop-mssa(licm))" < %s 2>/dev/null | FileCheck %s
+
+@g = external global i16
+
+define i16 @fn() {
+; CHECK-LABEL: define i16 @fn(
+; CHECK-SAME: ) #[[ATTR0:[0-9]+]] {
+; CHECK-NEXT:    [[V:%.*]] = load i16, ptr @g, align 2
+; CHECK-NEXT:    ret i16 [[V]]
+;
+  %v = load i16, ptr @g
+  ret i16 %v
+}
+
+declare void @fn2(i16)
+
+define void @test() {
+; CHECK-LABEL: define void @test() {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br label %[[LOOP:.*]]
+; CHECK:       [[LOOP]]:
+; CHECK-NEXT:    [[CALL:%.*]] = call i16 @fn(i32 0)
+; CHECK-NEXT:    call void @fn2(i16 [[CALL]])
+; CHECK-NEXT:    br i1 false, label %[[LOOP]], label %[[EXIT:.*]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %loop
+
+loop:
+  %call = call i16 @fn(i32 0) ; intentional signature mismatch
+  call void @fn2(i16 %call)
+  br i1 false, label %loop, label %exit
+
+exit:
+  ret void
+}