[PowerPC] Fix PR17155 - Ignore COPY_TO_REGCLASS during emit.

Fast-isel generates a COPY_TO_REGCLASS for widening f32 to f64, which
is a nop on PPC64.  This is needed to keep the register class system
happy, but on the fast-isel path it is not removed before emit as it
is for DAG select.  Ignore this op when emitting instructions.