Factor code from ABI_CallFunctionRR and GetWriteTrampoline into a helper, and fix a special case.

The special case is where the registers are actually to be swapped (i.e.
func(ABI_PARAM2, ABI_PARAM1); this was previously impossible but would
be ugly not to handle anyway.
Rationalize temporary register usage.

Rather than using a variety of registers including RSI, ABI_PARAM1
(either RCX or RDI), RCX, and RDX, the rule is:

- RDI and RSI are never used.  This allows them to be allocated on Unix,
bringing parity with Windows.

- RDX is a permanent temporary register along with RAX (and is thus not
FlushLocked).  It's used frequently enough that allocating it would
probably be a bad idea, as it would constantly get flushed.

- RCX is allocatable, but is flushed in two situations:
    - Non-immediate shifts (rlwnm), because x86 requires RCX to be used.
    - Paired single loads and stores, because they require three
    temporary registers: the helper functions take two integer
    arguments, and another register is used as an index to get the
    function address.
These should be relatively rare.

While we're at it, in stores, use the registers directly where possible
rather than always using temporaries (by making SafeWriteRegToReg
clobber less).  The address doesn't need to be clobbered in the usual
case, and on CPUs with MOVBE, neither does the value.

Oh, and get rid of a useless MEMCHECK.

This commit does not actually add new registers to the allocation order;
it is intended to test for any performance or correctness issues
separately.
Actually add RCX, plus RDI and RSI on Unix.

And switch to a register order that consistently prefers callee-save to
caller-save.  phire suggested putting rdi/rsi first, even though they're
caller-save, to save code space; this is more conservative and I can do
that later.
Symbolicize explicit uses of x86 registers where possible (GPRs only for now).

Uses are split into three categories:

- Arbitrary (except for size savings) - constants like RSCRATCH are
used.

- ABI (i.e. RAX as return value) - ABI_RETURN is used.

- Fixed by architecture (RCX shifts, RDX/RAX for some instructions) -
explicit register is kept.

In theory this allows the assignments to be modified easily.  I verified
that I was able to run Melee with all the registers changed, although
there may be issues if RSCRATCH[2] and ABI_PARAM{1,2} conflict.