nir/builder: Remove the use_fmov parameter from nir_swizzle

nir/builder: Remove the use_fmov parameter from nir_swizzle

This flag has caused more confusion than good in most cases.  You can
validly use imov for floats or fmov for integers because, without source
modifiers, neither modify their input in any way.  Using imov for floats
is more reliable so we go that direction.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>