[PowerPC] Preparatory refactoring for making prologue and epilogue
safe on PPC32 SVR4 ABI

[Patch and following text by Mark Minich; committing on his behalf.]

There are FIXME's in PowerPC/PPCFrameLowering.cpp, method
PPCFrameLowering::emitPrologue() related to "negative offsets of R1"
on PPC32 SVR4. They're true, but the real issue is that on PPC32 SVR4
(and any ABI without a Red Zone), no spills may be made until after
the stackframe is claimed, which also includes the LR spill which is
at a positive offset. The same problem exists in emitEpilogue(),
though there's no FIXME for it. I intend to fix this issue, making
LLVM-compiled code finally safe for use on SVR4/EABI/e500 32-bit
platforms (including in particular, OS-free embedded systems & kernel
code, where interrupts may share the same stack as user code).

In preparation for making these changes, to make the diffs for the
functional changes less cluttered, I am providing the non-functional
refactorings in two stages:

Stage 1 does some minor fluffy refactorings to pull multiple method
calls up into a single bool, creating named bools for repeated uses of
obscure logic, moving some code up earlier because either stage 2 or
my final version will require it earlier, and rewording/adding some
comments. My stage 1 changes can be characterized as primarily fluffy
cleanup, the purpose of which may be unclear until the stage 2 or
final changes are made.

My stage 2 refactorings combine the separate PPC32 & PPC64 logic,
which is currently performed by largely duplicate code, into a single
flow, with the differences handled by a group of constants initialized
early in the methods.

This submission is for my stage 1 changes. There should be no
functional changes whatsoever; this is a pure refactoring.