[CVP] Guard against poison in common phi value transform (PR50399)

The common phi value transform replaces constants with values that
have the same value as the constant on a given edge. However, LVI
generally only provides information that is correct up to poison,
so this can end up replacing a well-defined value with poison.
D69442 addressed an instance of this problem by clearing poison
flags on the generating instruction, which was sufficient at the
time. rGa917fb89dc28 made LVI's edge value analysis slightly more
powerful, and clearing poison flags is no longer sufficient.

This patch changes the transform to instead explicitly guard against
a poison value instead. This should be satisfied for most cases due
to a prior branch on poison.

Fixes https://bugs.llvm.org/show_bug.cgi?id=50399.

Differential Revision: https://reviews.llvm.org/D102966

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