glsl: Find the "closest" signature when there are multiple matches.

glsl: Find the "closest" signature when there are multiple matches.

Previously, ir_function::matching_signature had a fatal bug: if a
function had more than one non-exact match, it would simply return NULL.

This occured, for example, when looking for max(uvec3, uvec3):
- max(vec3, vec3)   -> score 1 (found first)
- max(ivec3, ivec3) -> score 1 (found second...used to return NULL here)
- max(uvec3, uvec3) -> score 0 (exact match...the right answer)

This did not occur for max(ivec3, ivec3) since the second match found
was an exact match.

The new behavior is to return a match with the lowest score.  If there
is an exact match, that will be returned.  Otherwise, a match with the
least number of implicit conversions is chosen.

Fixes piglit tests max-uvec3.vert and glsl-inexact-overloads.shader_test.

NOTE: This is a candidate for the 7.10 and 7.11 branches.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>