Reapply [SimplifyCFG] Make FoldCondBranchOnPHI more amenable to extension (NFCI)

Reapply with SmallMapVector instead of SmallDenseMap, which should
address the non-determinism issue.

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This general threading transform can be performed whenever we know
a constant value for the condition in a predecessor, which would
currently just be the case of a phi node with constant arguments.