[JumpThreading] ProcessBranchOnXOR(): bailout if any pred ends in indirect branch (PR46857)

SplitBlockPredecessors() can not split blocks that have such terminators,
and in two other places we already ensure that we don't end up calling
SplitBlockPredecessors() on such blocks. Do so in one more place.

Fixes https://bugs.llvm.org/show_bug.cgi?id=46857