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

[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: https://github.com/llvm/llvm-project/issues/58812

Reviewed By: nikic

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