Jit: FIFO optimization improvements

This introduces speculative constants, allowing FIFO writes to be
optimized in more places.

It also clarifies the guarantees of the FIFO optimization, changing
the location of some of the checks and potentially avoiding redundant
checks.

Source/Core/Core/PowerPC/CachedInterpreter.cpp

@@ -136,7 +136,7 @@ void CachedInterpreter::Jit(u32 address)
 
   js.blockStart = PC;
   js.firstFPInstructionFound = false;
-  js.fifoBytesThisBlock = 0;
+  js.fifoBytesSinceCheck = 0;
   js.downcountAmount = 0;
   js.curBlock = b;