glsl: Refactor is_zero/one/negative_one into an is_value() method.

glsl: Refactor is_zero/one/negative_one into an is_value() method.

This patch creates a new generic is_value() method, which checks if an
ir_constant has a particular value.  (For vectors, it must have the
single value repeated across all components.)

It then rewrites the is_zero/is_one/is_negative_one methods to use this
generic helper.  All three were basically identical except for the value
they checked for.  The other difference is that is_negative_one rejects
boolean types.  The new is_value function maintains this behavior, only
allowing boolean types when checking for 0 or 1.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>