In some cases the value of a block's branch predicate is correlated with the
predecessor of the block. Often this correlation is hinted at by the presence
of phis in the predicate's tree and/or phi VNs in in the predicate's VN graph.

For each predecessor of a block, we evaluate the predicate value number using
the values brought to the block by that predecessor. If we find correlations,
we use them to drive the existing jump threading optimization.

Also, if we end up partially disambiguating such that there is just one
remaining predecessor, update the value number of the predicate to reflect
the values that flow in from that predecessor.

Fixes dotnet#75944.
Contributes to dotnet#48115.

different SSA def numbers, despite the fact that we don't allow overlapping
SSA lifetimes. For example there may be a Phi in another block that gets
copied to some other local and then reaches the current block via that local.

So make sure that when we search local PHIs we also match the ssa def
number to ensure we're looking at the right PHIs.