glsl: Pack flat "varyings" of mixed types together.

This patch enhances the varying packing code so that flat varyings of
uint, int, and float types can be packed together.

We accomplish this in lower_packed_varyings.cpp by making the type of
all flat varyings ivec4, and then using information-preserving type
conversions (e.g. ir_unop_bitcast_f2i) to convert all other types to
ints.

The varying_matches::compute_packing_class() function is updated to
reflect the fact that varying packing no longer needs to segregate
varyings of different base types.

Fixes piglit test varying-packing-mixed-types.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

v2: Split lower_packed_varyings_visitor::bitwise_assign into
pack/unpack variants.
WIP: allow varying structs