Reapply [SimplifyCFG] Handle branch on same condition in pred more directly

Reapplying without changes, after a fix to a dependent patch.

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Rather than creating a PHI node and then using the PHI threading
code, directly handle this case in
FoldCondBranchOnValueKnownInPredecessor().

This change is supposed to be NFC-ish, but may cause changes due
to different transform order.