glsl: Add a gl_shader_program parameter to _mesa_uniform_{merge,split}_location_offset

glsl: Add a gl_shader_program parameter to _mesa_uniform_{merge,split}_location_offset

This will be used in the next commit.

NOTE: This is a candidate for stable release branches.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-and-tested-by: Chad Versace <chad.versace@linux.intel.com>
(cherry picked from commit 5097f358419c067a71e96e39764b3bb0a716bdbb)