aco: don't create sendmsg(dealloc_vgprs) if scratch is used

aco: don't create sendmsg(dealloc_vgprs) if scratch is used

LLVM does something similar: https://reviews.llvm.org/D153295

fossil-db (gfx1100):
Totals from 21 (0.02% of 133461) affected shaders:
Instrs: 147428 -> 147396 (-0.02%)
CodeSize: 797188 -> 797060 (-0.02%)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Fixes: 2930317cea53 ("aco/gfx11: deallocate VGPRs at the end of the shader")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24669>