glsl: tidy up validation loop in linker

There is no need to have a separate loop to determine the first stage in
the shader program. Previously there were other users of this but since
this is the last remain user this patch changes the code to simply detect
the first stage directly.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26747>

src/compiler/glsl/linker.cpp

@@ -2868,6 +2868,7 @@ link_shaders(struct gl_context *ctx, struct gl_shader_program *prog)
 #endif
 
    void *mem_ctx = ralloc_context(NULL); // temporary linker context
+   unsigned prev = MESA_SHADER_STAGES;
 
    /* Separate the shaders into groups based on their type.
     */
@@ -3032,17 +3033,6 @@ link_shaders(struct gl_context *ctx, struct gl_shader_program *prog)
    if (!prog->data->LinkStatus)
       goto done;
 
-   unsigned first, prev;
-
-   first = MESA_SHADER_STAGES;
-   /* Determine first and last stage. */
-   for (unsigned i = 0; i < MESA_SHADER_STAGES; i++) {
-      if (!prog->_LinkedShaders[i])
-         continue;
-      if (first == MESA_SHADER_STAGES)
-         first = i;
-   }
-
    check_explicit_uniform_locations(&ctx->Extensions, prog);
    link_assign_subroutine_types(prog);
    verify_subroutine_associated_funcs(prog);
@@ -3055,11 +3045,15 @@ link_shaders(struct gl_context *ctx, struct gl_shader_program *prog)
    /* Validate the inputs of each stage with the output of the preceding
     * stage.
     */
-   prev = first;
-   for (unsigned i = prev + 1; i <= MESA_SHADER_FRAGMENT; i++) {
+   for (unsigned i = 0; i <= MESA_SHADER_FRAGMENT; i++) {
       if (prog->_LinkedShaders[i] == NULL)
          continue;
 
+      if (prev == MESA_SHADER_STAGES) {
+         prev = i;
+         continue;
+      }
+
       validate_interstage_inout_blocks(prog, prog->_LinkedShaders[prev],
                                        prog->_LinkedShaders[i]);
       if (!prog->data->LinkStatus)