x64Emitter: Add the xmm, xmm form of PSRLQ instruction.
x64Emitter: Fix the PSUBQ instruction's opcode
Interpeter: Fixed ConvertToDouble to match the manual.

Also added some documntation comments.
Fixed DoubleToSingle in Jit64 and JitIL.

This method doesn't invlove messing around with the quriks of the x87 fpu and
should be reasonably fast. As a bonus, it does the correct thing for out
of range doubles.
Merge pull request #1 from phire/ppc_fp

Ppc fp