[GVN] teach ConstantFolding correct handling of non-integral addrspace casts

Here we teach the ConstantFolding analysis pass that it is not legal to
replace a load of a bitcast constant (having a non-integral addrspace)
with a bitcast of the value of that constant (with a different
non-integral addrspace).

But also teach it that certain bit patterns are always known and
convertable (a fact it already uses elsewhere). This required us to also
fix a globalopt test, since, after this change, LLVM is able to realize
that the test actually is a valid transform (NULL is always a known
bit-pattern) and so it doesn't need to emit the failure remarks for it.

Also simplify some of the negative tests for transforms by avoiding a
type change in their bitcast, and add positive versions of the same
tests, to show that they otherwise should work.

Differential Revision: https://reviews.llvm.org/D59730