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.