nir: Document a couple instances of parent_instr

nir_ssa_def::parent_instr and nir_src::parent_instr have the same name,
but they mean really different things.  I choose to save the next person
the hour+ that I just spent figuring that out.  Even now that I know, I
doubt I'd notice in code review that someone typed foo->parent_instr
when they actually meant foo->ssa->parent_instr.

v2: Minor wording tweak in nir_ssa_def::parent_instr.  Suggested by
Jason.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir.h

@@ -490,6 +490,7 @@ typedef struct nir_ssa_def {
    /** Index into the live_in and live_out bitfields */
    unsigned live_index;
 
+   /** Instruction which produces this SSA value. */
    nir_instr *parent_instr;
 
    /** set of nir_instrs where this register is used (read from) */
@@ -529,6 +530,7 @@ struct nir_if;
 
 typedef struct nir_src {
    union {
+      /** Instruction that consumes this value as a source. */
       nir_instr *parent_instr;
       struct nir_if *parent_if;
    };