nir: few missing struct names

nir.h is a bit inconsistent about 'typedef struct {} nir_foo' vs
'typedef struct nir_foo {} nir_foo'.  But missing struct name tags is
inconvenient when you need a fwd declaration without pulling in all
of nir.

So add missing struct name tag for nir_variable, and a couple other
spots where it would likely be useful.

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>

src/glsl/nir/nir.h

@@ -139,7 +139,7 @@ typedef enum {
  * ir_variable - it should be easy to translate between the two.
  */
 
-typedef struct {
+typedef struct nir_variable {
    struct exec_node node;
 
    /**
@@ -349,7 +349,7 @@ typedef struct {
 #define nir_foreach_variable(var, var_list) \
    foreach_list_typed(nir_variable, var, node, var_list)
 
-typedef struct {
+typedef struct nir_register {
    struct exec_node node;
 
    unsigned num_components; /** < number of vector components */
@@ -443,7 +443,7 @@ nir_instr_is_last(nir_instr *instr)
    return exec_node_is_tail_sentinel(exec_node_get_next(&instr->node));
 }
 
-typedef struct {
+typedef struct nir_ssa_def {
    /** for debugging only, can be NULL */
    const char* name;