[SimplifyCFG] FoldBranchToCommonDest: don't require that cmp of br is last instruction

There is no correctness need for that, and since we allow live-out
uses, this could theoretically happen, because currently nothing
will move the cond to right before the branch in those tests.
But regardless, lifting that restriction even makes the transform
easier to understand.

This makes the transform happen in 81 more cases (+0.55%)
)