[LoopPredication] Fix MemorySSA crash in predicateLoopExits

The attached testcase crashes without the patch (Not the same accesses
in the same order).

When we move instructions before another instruction, we also need to
update the memory accesses corresponding to it.

Reviewed-By: asbirlea
Differential Revision: https://reviews.llvm.org/D109197

Author: annamthomas

llvm/lib/Transforms/Scalar/LoopPredication.cpp

@@ -1127,6 +1127,10 @@ bool LoopPredication::predicateLoopExits(Loop *L, SCEVExpander &Rewriter) {
   // modifying the operand is legal.
   auto *IP = cast<Instruction>(WidenableBR->getCondition());
   IP->moveBefore(WidenableBR);
+  if (MSSAU)
+    if (auto *MUD = MSSAU->getMemorySSA()->getMemoryAccess(IP))
+       MSSAU->moveToPlace(MUD, WidenableBR->getParent(),
+                          MemorySSA::BeforeTerminator);
   Rewriter.setInsertPoint(IP);
   IRBuilder<> B(IP);
 

llvm/test/Transforms/LoopPredication/predicate-exits.ll

@@ -1082,6 +1082,34 @@ exit:
   ret i32 %result
 }
 
+define void @test_memssa() {
+bb:
+  %tmp = call i1 @llvm.experimental.widenable.condition()
+  %tmp1 = call i1 @llvm.experimental.widenable.condition()
+  br i1 %tmp, label %bb3, label %bb2
+
+bb2:                                              ; preds = %bb
+  unreachable
+
+bb3:                                              ; preds = %bb
+  br label %bb4
+
+bb4:                                              ; preds = %bb6, %bb3
+  %tmp5 = phi i32 [ %tmp7, %bb6 ], [ 0, %bb3 ]
+  br i1 undef, label %bb10, label %bb6
+
+bb6:                                              ; preds = %bb4
+  %tmp7 = add nuw nsw i32 %tmp5, 1
+  %tmp8 = icmp ult i32 %tmp7, undef
+  br i1 %tmp8, label %bb4, label %bb9
+
+bb9:                                              ; preds = %bb6
+  ret void
+
+bb10:                                             ; preds = %bb4
+  ret void
+}
+
 
 
 declare void @unknown()