nv50/ir: combine sequences of conversions

nv50/ir: combine sequences of conversions

In some cases shaders want non-default rounding when converting float to
integer. This can be done in one go, so merge the two ops. This comes up
in the packUnorm4x8 & co functions, as well as a few random shaders.
Overall shader-db impact is minimal, helping a handful of witcher2 and
other misc shaders.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>