nir: Remove nir_def::parent_instr

This reduces the footprint of nir_def by 8B on 64-bit systems.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@intel.com>
Acked-by: Marek Olšák <maraeo@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38313>

src/compiler/nir/nir.c

@@ -1663,7 +1663,6 @@ nir_def_init(nir_instr *instr, nir_def *def,
              unsigned num_components,
              unsigned bit_size)
 {
-   def->parent_instr = instr;
    list_inithead(&def->uses);
    def->num_components = num_components;
    def->bit_size = bit_size;

src/compiler/nir/nir.h

@@ -1002,9 +1002,6 @@ nir_instr_is_last(const nir_instr *instr)
 }
 
 typedef struct nir_def {
-   /** Instruction which produces this SSA value. */
-   nir_instr *parent_instr;
-
    /** set of nir_instrs where this register is used (read from) */
    struct list_head uses;