patch: remove PseudoSourceValue from Value.

[Nick Lewycky]

The attached patch removes PseudoSourceValue from the Value hierarchy. No
functionality change intended, but I had to make a number of assumptions in
order to make that claim. I assume nobody ever assigned names to PSV's. I
assume nobody has an AliasAnalysis that is smart about PSV's (it's not in
tree). That sort of thing.

I'm going to say up front, this patch isn't entirely pretty. There's some
PointerUnions involved. Some code did get nicer, there were users that had
tests for Value* then if isa<PseudoSourceValue> do something that would end
in an unconditional return or continue, then afterwards handle non-PSV
Value's. These are now sensible conditions for the two cases. Some code got
worse. Target/Mips wins the contest for actually having code that treated
PSV's and Value's through the same code paths, good job guys! Sorry that
I'm doing this to you.

At its core, this patch makes PseudoSourceValue the root of its own
hierarchy with just itself and FixedPseudoSourceValue in it, then updated
MachinePointerInfo to store a pointer-union of the PSV and Value*. Various
bits of code throughout lib/CodeGen and lib/Target are updated to pass
MPI's around instead of extracting the Value* from an MPI, passing that,
then rebuilding the MPI on the other end. Such code was silly anyways.

Please review. Before we get a low-level code review, I'm expecting to be
told that I've violated some design criteria of the backend that I wasn't
aware of. I mean, I moved a method from private to public, so that's a
pretty big sign. Please feel free to tell me how things ought to be layered.

Nick
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