mesa: Replace _mesa_program_index_to_target with _mesa_shader_stage_to_program.

In my recent zeal to refactor Mesa's handling of the gl_shader_stage
enum, I accidentally wound up with two functions that do the same
thing: _mesa_program_index_to_target(), and
_mesa_shader_stage_to_program().

This patch keeps _mesa_shader_stage_to_program(), since its name is
more consistent with other related functions.  However, it changes the
signature so that it accepts an unsigned integer instead of a
gl_shader_stage--this avoids awkward casts when the function is called
from C++ code.