pan/bi: Add a bi_instr::blend_target

On Bifrost, we have to return the blend return offset in the compiled
shader info and that means we need to be able to index into an array by
blend target deep inside the compiler.  Instead of assuming bound blend
targets and subtracting BIR_FAU_BLEND_0 from fau_idx, add a separte
blend_target to bi_instr and use that.  This way what we return will be
based on the nir_io_semantics::location, regardless of where the actual
blend descriptor comes from.

Reviewed-by: Lars-Ivar Hesselberg Simonsen <lars-ivar.simonsen@arm.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/39274>

src/panfrost/compiler/bifrost/bifrost/bi_pack.c

@@ -693,7 +693,7 @@ bi_collect_blend_ret_addr(bi_context *ctx, struct util_dynarray *emission,
    if (!ins || ins->op != BI_OPCODE_BLEND)
       return;
 
-   unsigned loc = tuple->regs.fau_idx - BIR_FAU_BLEND_0;
+   unsigned loc = ins->blend_target;
    assert(loc < ARRAY_SIZE(ctx->info.bifrost->blend));
    assert(!ctx->info.bifrost->blend[loc].return_offset);
    ctx->info.bifrost->blend[loc].return_offset =

src/panfrost/compiler/bifrost/bifrost_compile.c

@@ -2168,15 +2168,16 @@ bi_emit_intrinsic(bi_builder *b, nir_intrinsic_instr *instr)
          bi_make_vec_to(b, rgba, srcs, channels, 4, size);
       }
 
-      bi_blend_to(b, bi_temp(b->shader), rgba, coverage,
-                  bi_extract(b, desc, 0), bi_extract(b, desc, 1),
-                  rgba2, regfmt, sr_count, sr_count_2);
-
       nir_io_semantics io = nir_intrinsic_io_semantics(instr);
       assert(io.location >= FRAG_RESULT_DATA0);
       assert(io.location <= FRAG_RESULT_DATA7);
       unsigned rt = io.location - FRAG_RESULT_DATA0;
 
+      bi_instr *I = bi_blend_to(b, bi_temp(b->shader), rgba, coverage,
+                                bi_extract(b, desc, 0), bi_extract(b, desc, 1),
+                                rgba2, regfmt, sr_count, sr_count_2);
+      I->blend_target = rt;
+
       b->shader->info.bifrost->blend[rt].type = T;
       if (instr->intrinsic == nir_intrinsic_blend2_pan) {
          assert(b->shader->info.bifrost->blend_src1_type == nir_type_invalid);

src/panfrost/compiler/bifrost/compiler.h

@@ -629,6 +629,7 @@ typedef struct {
       bool format;               /* LEA_TEX */
       bool z_stencil;            /* LD_TILE */
       bool scheduling_barrier;   /* NOP */
+      unsigned blend_target;     /* BLEND */
 
       struct {
          enum bi_special special;   /* FADD_RSCALE, FMA_RSCALE */