glsl/nir: int constants as float for native_integers=false

glsl/nir: int constants as float for native_integers=false

All alu instructions emitted with native_integers=false expect float
(or bool in some cases) constants, so this change is necessary.

This will cause changes with some intrinsics which had integer sources,
such as nir_intrinsic_load_uniform. Apparently it might cause issues with
some opt passes, but perhaps those don't apply in OpenGL ES 2.0 cases?

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>