[NFCI][SimplifyCFG] simplifyCondBranch(): assert that branch is non-tautological

We really shouldn't deal with a conditional branch that can be trivially
constant-folded into an unconditional branch.

Indeed, barring failure to trigger BB reprocessing, that should be true,
so let's assert as much, and hope the assertion never fires.
If it does, we have a bug to fix.