glsl: Add a lowering pass for packing varyings.

glsl: Add a lowering pass for packing varyings.

This lowering pass generates GLSL code that manually packs varyings
into vec4 slots, for the benefit of back-ends that don't support
packed varyings natively.

No functional change--the lowering pass is not yet used.

Reviewed-by: Eric Anholt <eric@anholt.net>
v2: Don't use ir_hierarchical_visitor--just loop over instructions
directly.  Also, make the names of the packed varyings include the
names of the original varyings that were packed into them.