[AMDGPU] refactor WQM pass in preparation for WWM (NFCI)

[AMDGPU] refactor WQM pass in preparation for WWM (NFCI)

Summary:
Right now, the WQM pass conflates two different things when tracking the
Needs of an instruction:

1. Needs can be StateWQM, which is propagated to other instructions, and
means that this instruction (and everything it depends on) must be
calculated in WQM.
2. Needs can be StateExact, which is not propagated to other
instructions, and means that this instruction must not be calculated in
WQM and WQM-ness must not be propagated past this instruction.

This works now because there are only two different states, but in the
future we want to be able to express things like "calculate this in WQM,
but please disable WWM and don't propagate it" (to implement
@llvm.amdgcn.set.inactive). In order to do this, we need to split the
per-instruction Needs field in two: a new Needs field, which can only
contain StateWQM (and in the future, StateWWM) and is propagated to
sources, and a Disables field, which can also contain just StateWQM or
nothing for now.

We keep the per-block tracking the same for now, by translating
Needs/Disables to the old representation with only StateWQM or
StateExact. The other place that needs special handling is when we
emit the state transitions. We could just translate back to the old
representation there as well, which we almost do, but instead of 0 as a
placeholder value for "any state," we explicitly or together all the
states an instruction is allowed to be in. This lets us refactor the
code in preparation for WWM, where we'll need to be able to handle
things like "this instruction must be in Exact or WQM, but not WWM."

Reviewers: arsenm, nhaehnle, tpr

Subscribers: kzhuravl, wdng, yaxunl, dstuttard, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D35523

llvm-svn: 310086