aco: allow reading/writing upper halves/bytes when possible

Use SDWA, opsel or a different opcode to achieve this.

shader-db (Navi, fp16 enabled):
Totals from 42 (0.03% of 127638) affected shaders:
VGPRs: 3424 -> 3416 (-0.23%)
CodeSize: 811124 -> 811984 (+0.11%); split: -0.12%, +0.23%
Instrs: 156638 -> 155733 (-0.58%)
Cycles: 1994180 -> 1982568 (-0.58%); split: -0.59%, +0.00%
VMEM: 7019 -> 7187 (+2.39%); split: +3.45%, -1.05%
SMEM: 1771 -> 1770 (-0.06%); split: +0.06%, -0.11%
VClause: 1477 -> 1475 (-0.14%)
Copies: 13216 -> 12406 (-6.13%)
Branches: 5942 -> 5901 (-0.69%)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5040>

src/amd/compiler/aco_optimizer.cpp

@@ -490,7 +490,7 @@ bool can_swap_operands(aco_ptr<Instruction>& instr)
    }
 }
 
-bool can_use_VOP3(opt_ctx& ctx, aco_ptr<Instruction>& instr)
+bool can_use_VOP3(opt_ctx& ctx, const aco_ptr<Instruction>& instr)
 {
    if (instr->isVOP3())
       return true;