nir/tex_instr_create: Initialize all 4 sources

This helps a lot with things like lowering passes that may need to add
sources.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>

src/glsl/nir/nir.c

@@ -461,7 +461,7 @@ nir_tex_instr_create(void *mem_ctx, unsigned num_srcs)
    dest_init(&instr->dest);
 
    instr->num_srcs = num_srcs;
-   for (unsigned i = 0; i < num_srcs; i++)
+   for (unsigned i = 0; i < 4; i++)
       src_init(&instr->src[i]);
 
    instr->has_predicate = false;