Use it in `printVRegOrUnit()`, `getPressureSets()`/`PSetIterator`,
and in functions/classes dealing with register pressure.

Static type checking revealed several bugs, mainly in MachinePipeliner.
I'm not very familiar with this pass, so I left a bunch of FIXMEs.

There is one bug in `findUseBetween()` in RegisterPressure.cpp, also
annotated with a FIXME.

In both ScheduleDAGInstrs and MachineScheduler, we call `BufferSize = 0`
as _reserved_ and `BufferSize = 1` as _unbuffered_. This convention is
stem from the fact that we set `SUnit::hasReservedResource` to true when
any of the SUnit's consumed resources has BufferSize equal to zero; set
`SUnit::isUnbuffered` to true when any of its consumed resources has
BufferSize equal to one.

However, `SchedBoundary::isUnbufferedGroup` doesn't really follow this
convention: it returns true when the resource in question is a
`ProcResGroup` and its BufferSize equals to **zero** rather than one.
This could be really confusing for the reader. This patch renames this
function to `isReservedGroup` in aligned with the convention mentioned
above.

NFC.

Identified with modernize-use-equals-default.

This reverts commit a7414796c0854a9e6f649d922a58aa63147ae2e4.

This breaks builds:
 3355 |            << SchedModel->getResourceName(CurrZone.getZoneCritResIdx()) << "\n";
      |               ~~~~~~~~~~  ^
/home/buildbot/workspace/bolt-aarch64-ubuntu-clang/llvm-project/llvm/lib/CodeGen/MachineScheduler.cpp:3358:51: error: no member named 'getResourceName' in 'llvm::TargetSchedModel'
 3358 |     LDBG() << "  RemainingLimit: " << SchedModel->getResourceName(OtherCritIdx)
      |                                       ~~~~~~~~~~  ^
2 errors generated.

E.g. https://lab.llvm.org/buildbot/#/builders/128/builds/7522

These lines are heavily skewed and hard to read. Using the new LDBG
there instead.

NFC.

It is unnecessary and confusing to have a do/while loop that checks
SU->isScheduled as this should never be true.

ScheduleDAGMI::updateQueues() is always called after pickNode() and it
sets isScheduled on the SU. Turn this into an assertion instead.

This patch updates `overrideSchedPolicy` and `overridePostRASchedPolicy`
to take a
`SchedRegion` parameter instead of just `NumRegionInstrs`. This provides
access to both the
instruction range and the parent `MachineBasicBlock`, which enables
looking up function-level
attributes.

With this change, targets can select post-RA scheduling direction per
function using a function
attribute. For example:

```cpp
void overridePostRASchedPolicy(MachineSchedPolicy &Policy,
                               const SchedRegion &Region) const {
  const Function &F = Region.RegionBegin->getMF()->getFunction();
  Attribute Attr = F.getFnAttribute("amdgpu-post-ra-direction");
  ...
}

std::tie clearly expresses the intent while slightly shortening the
code.