This was being used for 2 different purposes.

The TargetMachine constructor prepends +64bit based on isPPC64
triples as a mode switch. The same feature name was also explicitly
added to different processors, making it impossible to perform a pure
feature check for whether 64-bit mode is enabled ir not. i.e.,
checkFeatures("+64bit") would be true even for ppc32 triples.

The comment in tablegen suggests it's relevant to track which processors
support 64-bit mode independently of whether that's the active compile
target, so replace that with a new feature.

This already exists in the base class.

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");
  ...
}

#119969 adds a couple of new methods to this class, which will need to
be overridden by these targets.

Part of #119709.

Pull Request: https://github.com/llvm/llvm-project/pull/119968

Identified with misc-include-cleaner.

Utilize common API in PPCTargetParser
(https://github.com/llvm/llvm-project/pull/97541) to set default CPU
with same interfaces for LLC.
This will update AIX default CPU to pwr7 and LoP powerppc64 default CPU
to ppc64.

Some targets only pass a TargetMachine & to their subtarget constructor
and require a static_cast to their target-specific TargetMachine subclass
to create *InstructionSelector.

These 3 targets already have the correct TargetMachine subclass
reference so no cast is needed.

Replace it with a forward declaration instead. Analysis.h is pulled in
by all passes, but not all passes need to access the module.

This removes the uses of target flags to disable subreg liveness,
relying on the `-enable-subreg-liveness` flag instead. The
`-enable-subreg-liveness` flag has been changed to take precedence over
the subtarget if set, and one use of `Subtarget->enableSubRegLiveness()`
has been changed to `MRI->subRegLivenessEnabled()` to make sure the
option properly applies.