nir: Add nir_instr_is_before helper

is_instr_between works similarly but requires special-casing w.r.t
the first instruction in a block for the same effect.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29577>

src/compiler/nir/nir.c

@@ -1602,6 +1602,24 @@ nir_def_rewrite_uses_src(nir_def *def, nir_src new_src)
    nir_def_rewrite_uses(def, new_src.ssa);
 }
 
+bool
+nir_instr_is_before(nir_instr *first, nir_instr *second)
+{
+   if (first->block != second->block)
+      return false;
+
+   /* Search backwards looking for "first" */
+   while (second != nir_block_first_instr(second->block)) {
+      second = nir_instr_prev(second);
+      assert(second);
+
+      if (first == second)
+         return true;
+   }
+
+   return false;
+}
+
 static bool
 is_instr_between(nir_instr *start, nir_instr *end, nir_instr *between)
 {

src/compiler/nir/nir.h

@@ -5098,6 +5098,9 @@ void nir_instr_clear_src(nir_instr *instr, nir_src *src);
 
 void nir_instr_move_src(nir_instr *dest_instr, nir_src *dest, nir_src *src);
 
+/** Returns true if first comes before second in a block. */
+bool nir_instr_is_before(nir_instr *first, nir_instr *second);
+
 void nir_def_init(nir_instr *instr, nir_def *def,
                   unsigned num_components, unsigned bit_size);
 static inline void