[JT] check xor operand is exactly the same in processBranchOnXOR

Reproducer:

    ; RUN: opt -S -jump-threading < %s
    define void @test() {
    entry:
    br i1 false, label %loop, label %exit

    loop:
    %bool = phi i1 [ %xor, %loop.latch ], [ false, %entry ]
    %cmp = icmp eq i16 0, 1
    %xor = xor i1 %cmp, %bool
    br i1 %bool, label %loop.latch, label %exit

    loop.latch:
    %dummy = phi i16 [ 0, %loop ]
    br label %loop

    exit:
    ret void
    }

On this occassion, phi node %bool is actually %xor, and doing substitution causes assertion failure.

Fixes: #58812

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D139783

llvm/lib/Transforms/Scalar/JumpThreading.cpp

@@ -1931,7 +1931,7 @@ bool JumpThreadingPass::processBranchOnXOR(BinaryOperator *BO) {
       // If all preds provide undef, just nuke the xor, because it is undef too.
       BO->replaceAllUsesWith(UndefValue::get(BO->getType()));
       BO->eraseFromParent();
-    } else if (SplitVal->isZero()) {
+    } else if (SplitVal->isZero() && BO != BO->getOperand(isLHS)) {
       // If all preds provide 0, replace the xor with the other input.
       BO->replaceAllUsesWith(BO->getOperand(isLHS));
       BO->eraseFromParent();

llvm/test/Transforms/JumpThreading/phi-xor-branch.ll

@@ -0,0 +1,33 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -passes=jump-threading %s | FileCheck %s
+
+; https://github.com/llvm/llvm-project/issues/58812
+
+define void @test() {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[EXIT:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[DUMMY:%.*]] = phi i16 [ 0, [[LOOP:%.*]] ]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i16 0, 1
+; CHECK-NEXT:    [[XOR:%.*]] = xor i1 false, [[XOR]]
+; CHECK-NEXT:    br i1 [[XOR]], label [[LOOP]], label [[EXIT]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br i1 false, label %loop, label %exit
+
+loop:
+  %bool = phi i1 [ %xor, %loop.latch ], [ false, %entry ]
+  %cmp = icmp eq i16 0, 1
+  %xor = xor i1 %cmp, %bool
+  br i1 %bool, label %loop.latch, label %exit
+
+loop.latch:
+  %dummy = phi i16 [ 0, %loop ]
+  br label %loop
+
+exit:
+  ret void
+}