[SimplifyCFG] performBranchToCommonDestFolding(): require block-closed SSA form for bonus instructions (PR51125)

I can't seem to wrap my head around the proper fix here,
we should be fine without this requirement, iff we can form this form,
but the naive attempt (https://reviews.llvm.org/D106317) has failed.
So just to unblock the release, put up a restriction.

Fixes https://bugs.llvm.org/show_bug.cgi?id=51125

Author: LebedevRI

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -1095,17 +1095,24 @@ static void CloneInstructionsIntoPredecessorBlockAndUpdateSSAUses(
 
     // Update (liveout) uses of bonus instructions,
     // now that the bonus instruction has been cloned into predecessor.
-    SSAUpdater SSAUpdate;
-    SSAUpdate.Initialize(BonusInst.getType(),
-                         (NewBonusInst->getName() + ".merge").str());
-    SSAUpdate.AddAvailableValue(BB, &BonusInst);
-    SSAUpdate.AddAvailableValue(PredBlock, NewBonusInst);
+    // Note that we expect to be in a block-closed SSA form for this to work!
     for (Use &U : make_early_inc_range(BonusInst.uses())) {
       auto *UI = cast<Instruction>(U.getUser());
-      if (UI->getParent() != PredBlock)
-        SSAUpdate.RewriteUseAfterInsertions(U);
-      else // Use is in the same block as, and comes before, NewBonusInst.
-        SSAUpdate.RewriteUse(U);
+      auto *PN = dyn_cast<PHINode>(UI);
+      if (!PN) {
+        assert(UI->getParent() == BB && BonusInst.comesBefore(UI) &&
+               "If the user is not a PHI node, then it should be in the same "
+               "block as, and come after, the original bonus instruction.");
+        continue; // Keep using the original bonus instruction.
+      }
+      // Is this the block-closed SSA form PHI node?
+      if (PN->getIncomingBlock(U) == BB)
+        continue; // Great, keep using the original bonus instruction.
+      // The only other alternative is an "use" when coming from
+      // the predecessor block - here we should refer to the cloned bonus instr.
+      assert(PN->getIncomingBlock(U) == PredBlock &&
+             "Not in block-closed SSA form?");
+      U.set(NewBonusInst);
     }
   }
 }
@@ -3239,6 +3246,17 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI, DomTreeUpdater *DTU,
     // Early exits once we reach the limit.
     if (NumBonusInsts > BonusInstThreshold)
       return false;
+
+    auto IsBCSSAUse = [BB, &I](Use &U) {
+      auto *UI = cast<Instruction>(U.getUser());
+      if (auto *PN = dyn_cast<PHINode>(UI))
+        return PN->getIncomingBlock(U) == BB;
+      return UI->getParent() == BB && I.comesBefore(UI);
+    };
+
+    // Does this instruction require rewriting of uses?
+    if (!all_of(I.uses(), IsBCSSAUse))
+      return false;
   }
 
   // Ok, we have the budget. Perform the transformation.

llvm/test/CodeGen/Thumb2/mve-float16regloops.ll

@@ -1054,7 +1054,7 @@ define void @fir(%struct.arm_fir_instance_f32* nocapture readonly %S, half* noca
 ; CHECK-NEXT:    cmp r3, #8
 ; CHECK-NEXT:    str r1, [sp, #16] @ 4-byte Spill
 ; CHECK-NEXT:    blo.w .LBB16_12
-; CHECK-NEXT:  @ %bb.1: @ %entry
+; CHECK-NEXT:  @ %bb.1: @ %if.then
 ; CHECK-NEXT:    lsrs.w r12, r3, #2
 ; CHECK-NEXT:    beq.w .LBB16_12
 ; CHECK-NEXT:  @ %bb.2: @ %while.body.lr.ph

llvm/test/CodeGen/Thumb2/mve-float32regloops.ll

@@ -1048,7 +1048,7 @@ define void @fir(%struct.arm_fir_instance_f32* nocapture readonly %S, float* noc
 ; CHECK-NEXT:    sub sp, #24
 ; CHECK-NEXT:    cmp r3, #8
 ; CHECK-NEXT:    blo.w .LBB16_12
-; CHECK-NEXT:  @ %bb.1: @ %entry
+; CHECK-NEXT:  @ %bb.1: @ %if.then
 ; CHECK-NEXT:    lsrs.w r12, r3, #2
 ; CHECK-NEXT:    beq.w .LBB16_12
 ; CHECK-NEXT:  @ %bb.2: @ %while.body.lr.ph