JIT: fix snan/qnan handling in float loads/stores

Also simplify the conversion code with some suggestions by flacs; might even
be slightly faster now despite handling more cases.
Idle loop in GPU loop on dual core systems.

Previously we had decided to busy loop on systems due to Windows' scheduler being terrible and moving us around CPU cores when we yielded.
Along with context switching being a hot spot.
We had decided to busy loop in these situations instead, which allows us greater CPU performance on the video thread.
This can be attributed to multiple things, CPU not downclocking while busy looping, context switches happening less often, yielding taking more time
than a busy loop, etc.

One thing we had considered when moving over to a busy loop is the issues that dual core systems would now face due to Dolphin eating all of their CPU
resources. Effectively we are starving a dual core system of any time to do anything else due to the CPU thread always being pinned at 100% and then
the GPU thread also always at 100% just spinning around. We noted the potential for a performance regression, but dismissed it as most computers are
now becoming quad core or higher.

This change in particular has performance advantages on the dual core Nvidia Denver due to its architecture being nonstandard. If both CPU cores are
maxed out, the CPU can't effectively take any idle time to recompile host code blocks to its native VLIW architecture.
It can still do so, but it does less frequently which results in performance issues in Dolphin due to most code just running through the in-order
instruction decoder instead of the native VLIW architecture.
In one particular example, yielding moves the performance from 35-40FPS to 50-55FPS. So it is far more noticeable on Denver than any other system.
Of course once a triple or quad core Denver system comes out this will no longer be an issue on this architecture since it'll have a free core to do
all of this work.
Merge pull request #1623 from FioraAeterna/fixppcfp

JIT: fix PPC_FP snan/qnan handling
Merge pull request #1665 from Sonicadvance1/yielding_the_dual_core_world

Idle loop in GPU loop on dual core systems.
JIT: fix MOVD_xmm typoes in ConvertDoubleToSingle

Ordering, plus MOVQ instead of MOVD
Merge pull request #1677 from FioraAeterna/fixtypo

JIT: fix MOVD_xmm order typo in ConvertDoubleToSingle
VideoCommon: rewrite DataReader
VideoCommon: fifo cleanups
VideoCommon: cleanup OpcodeDecoder
VertexLoader: Move the old Datareader function into VertexLoader
VideoCommon: Rename s_pCurBufferPointer
VideoCommon: clean up VertexLoader
UnitTests: fix VertexLoaderTest