PowerPC: Simplify FADD in round-to-zero mode.

As part of the the sequence generated to implement long double -> int
conversions, we need to perform an FADD in round-to-zero mode.  This is
problematical since the FPSCR is not at all modeled at the SelectionDAG
level, and thus there is a risk of getting floating point instructions
generated out of sequence with the instructions to modify FPSCR.

The current code handles this by somewhat "special" patterns that in part
have dummy operands, and/or duplicate existing instructions, making them
awkward to handle in the asm parser.

This commit changes this by leaving the "FADD in round-to-zero mode"
as an atomic operation on the SelectionDAG level, and only split it up into
real instructions at the MI level (via custom inserter).  Since at *this*
level the FPSCR *is* modeled (via the "RM" hard register), much of the
"special" stuff can just go away, and the resulting patterns can be used by
the asm parser.

No significant change in generated code expected.
PowerPC: Simplify handling of fixups.

MCTargetDesc/PPCMCCodeEmitter.cpp current has code like:

 if (isSVR4ABI() && is64BitMode())
   Fixups.push_back(MCFixup::Create(0, MO.getExpr(),
                                    (MCFixupKind)PPC::fixup_ppc_toc16));
 else
   Fixups.push_back(MCFixup::Create(0, MO.getExpr(),
                                    (MCFixupKind)PPC::fixup_ppc_lo16));

This is a problem for the asm parser, since it requires knowledge of
the ABI / 64-bit mode to be set up.  However, more fundamentally,
at this point we shouldn't make such distinctions anyway; in an assembler
file, it always ought to be possible to e.g. generate TOC relocations even
when the main ABI is one that doesn't use TOC.

Fortunately, this is actually completely unnecessary; that code was added
to decide whether to generate TOC relocations, but that information is in
fact already encoded in the VariantKind of the underlying symbol.

This commit therefore merges those fixup types into one, and then decides
which relocation to use based on the VariantKind.

No changes in generated code.