[VPlan] Fix find last reductions not using frozen condition in select (#223962)
A find last reduction is something like:
(select c[n], x[n], (select c[n-1], x[n-1], (select ...)))
So even if previous iterations had poison for c[x], the top-level select
blocks it.
MaskSelect uses the non-frozen Cond, and that in turn is fed into
VPInstruction::ExtractLastActive, which in turn gets expanded to
vector.reduce.umax, which returns poison if any of the lanes were
poison.
So in the case e.g. c[n] = 1, c[n-1] = poison and VF=2, the scalar loop
returns x[n] whilst vector loop returns poison.
We're also introducing multiple uses of Cond which isn't sound for
undef, so for these reasons we need to use the frozen Cond.
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