i965/fs: Consider type mismatches in saturate propagation.

i965/fs: Consider type mismatches in saturate propagation.

NIR considers bcsel to produce and consume unsigned types, leading to
SEL instructions operating on unsigned types when the data is really
floating-point. Previous to this patch, saturate propagation would
happily transform

   (+f0) sel      g20:UD, g30:UD, g40:UD
         mov.sat  g50:F,  g20:F

into

   (+f0) sel.sat  g20:UD, g30:UD, g40:UD
         mov      g50:F,  g20:F

But since the meaning of .sat is dependent on the type of the
destination register, this is not valid.

Instead, allow saturate propagation to change the types of dest/source
on instructions that are simply copying data in order to propagate the
saturate modifier.

Fixes bad code gen in 158 programs.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>