[ConstantFolding] Unify handling of load from uniform value

There are a number of places that specially handle loads from a
uniform value where all the bits are the same (zero, one, undef,
poison), because we a) don't care about the load offset in that
case b) it bypasses casts that might not be legal generally but
do work with uniform values.

We had multiple implementations of this, with a different set of
supported values each time. This replaces two usages with a more
complete helper. Other usages will be replaced separately, because
they have larger impact.

This is part of D115924.