[llvm-dev] Use list waymarking bits

[Craig Topper via llvm-dev]

The documentation for the Use list waymarking bits says that we use the 2
lsbs of the Prev pointer in Use objects and the lsb of the User object for
the special bits. PointerLikeTypeTraits for both of these types indicates
they have 3 low bits available on 64-bit targets. We use PointerIntPair
which tries to use the highest low bits to store the integer. So we're
really using bits 2:1 of Use and bit 2 of User.

Should we fix the documentation or override the type traits on these to
match the documentation.

I'm particularly curious about the "portable trick" mentioned here
http://llvm.org/docs/ProgrammersManual.html#tagging-considerations

~Craig
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