compiler/glsl: refactor empty_uniform_block utilities to linker_util

This includes:
  * Move the defition of empty_uniform_block to linker_util.h
  * Move find_empty_block (with a rename) to linker_util.h
  * Refactor some code at linker.cpp to a new method at linker_util.h
    (link_util_update_empty_uniform_locations)

So all that code could be used by the GLSL linker and the NIR linker
used for ARB_gl_spirv.

v2: include just "ir_uniform.h" (Timothy Arceri)

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/compiler/glsl/linker.h

@@ -194,17 +194,4 @@ private:
                   const glsl_struct_field *named_ifc_member);
 };
 
-/**
- * Sometimes there are empty slots left over in UniformRemapTable after we
- * allocate slots to explicit locations. This struct represents a single
- * continouous block of empty slots in UniformRemapTable.
- */
-struct empty_uniform_block {
-   struct exec_node link;
-   /* The start location of the block */
-   unsigned start;
-   /* The number of slots in the block */
-   unsigned slots;
-};
-
 #endif /* GLSL_LINKER_H */