Do not set the rounding mode before exact floating-point operations.

Setting the rounding mode on some host architectures (including x86, according to <http://www.mega-nerd.com/FPcast/>) empties the floating-point unit's pipeline, resulting in poorer performance. Omitting the rounding mode before exact operations restores some performance.

Affected opcodes are CVT.W.S, CVT.W.D, CVT.L.S, CVT.L.D, ABS.S, MOV.S, NEG.S, ABS.D, MOV.D and NEG.D.
Do not set the rounding mode before floating-point widening conversions.

The rounding mode is completely unneeded when converting from W (32-bit integer) or S (32-bit floating-point) to D (64-bit floating-point), and is actually unused on MIPS processors.

Quoting from the MIPS Programmer's Manual, volume 2, for CVT.D.fmt:
The value in FPR fs, in format fmt, is converted to a value in double floating point format and rounded according to the current rounding mode in FCSR. The result is placed in FPR fd. If fmt is S or W, then the operation is always exact.
Remove a native test and branch from inter-block jumps.
Merge pull request #4 from Nebuleon/master

Optimisations.