glsl/ast: Explicitly track the set of case labels that occur after default

glsl/ast: Explicitly track the set of case labels that occur after default

Previously the instruction stream was walked looking for comparisons
with case-label values.  This should generate nearly identical code.
For at least fs-default-notlast-fallthrough.shader_test, the code is
identical.

This change will make later changes possible.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>