glsl: Avoid cascading errors when looking for a scalar boolean and failing.

glsl: Avoid cascading errors when looking for a scalar boolean and failing.

By always using a boolean, we should generally avoid further
complaints.  The failure case I see is logic_not, where the user might
understandably make the mistake of using `!' on a boolean vector (like
a piglit case did recently!), and then get a further complaint that
the new boolean type doesn't match the bvec it gets assigned to.

Fixes invalid-logic-not-06.vert (assertion failure when the bad type
ends up in an expression and ir_constant_expression gets angry).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=33314
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>