aco/optimizer_postRA: Don't assume all operand registers were written by same instr.

This assumption is no longer true since the post-RA optimizer
can work across blocks. It is now possible that some control
flow paths overwrite some but not all registers of an operand.

This commit may prevent invalid optimizations and/or assertion
failures (on debug builds).

Fossil DB stats unaffected on Navi 21.

Fixes: 0e4747d3fb
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18488>
(cherry picked from commit 2eab413cf7)

.pick_status.json

@@ -2578,7 +2578,7 @@
         "description": "aco/optimizer_postRA: Don't assume all operand registers were written by same instr.",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "0e4747d3fb7ec15f8c1d6b971b1352249e7d95c6"
     },

src/amd/compiler/aco_optimizer_postRA.cpp

@@ -127,16 +127,7 @@ last_writer_idx(pr_opt_ctx& ctx, const Operand& op)
    if (op.isConstant() || op.isUndefined())
       return const_or_undef;
 
-   assert(op.physReg().reg() < max_reg_cnt);
-   Idx instr_idx = ctx.instr_idx_by_regs[ctx.current_block->index][op.physReg().reg()];
-
-#ifndef NDEBUG
-   /* Debug mode:  */
-   instr_idx = last_writer_idx(ctx, op.physReg(), op.regClass());
-   assert(instr_idx != written_by_multiple_instrs);
-#endif
-
-   return instr_idx;
+   return last_writer_idx(ctx, op.physReg(), op.regClass());
 }
 
 bool