glsl: Fix broken constant expression handling for <, <=, >, and >=.

glsl: Fix broken constant expression handling for <, <=, >, and >=.

We were looping over all the vector components, but only dealing with
the first one.  This was masked by the fact that constant expression
handling on built-ins went through custom code for the lessThan()
/function/ rather than the ir_binop_less expression operator.

NOTE: This is a candidate for all release branches.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Olivier Galibert <galibert@pobox.com>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>