ir3: Consider const alloc alignment in free space size calcs

The alignment was considered only for offset, but its users
(at least ir3_nir_opt_preamble) expect the size itself to also
be aligned.

Fixes tests:
  dEQP-VK.spirv_assembly.instruction.graphics.16bit_storage.struct_mixed_types.uniform_buffer_block_geom
  dEQP-VK.spirv_assembly.instruction.graphics.16bit_storage.struct_mixed_types.uniform_buffer_block_tessc
  dEQP-VK.spirv_assembly.instruction.graphics.16bit_storage.struct_mixed_types.uniform_buffer_block_tesse
  gmem-dEQP-VK.spirv_assembly.instruction.graphics.16bit_storage.struct_mixed_types.uniform_buffer_block_tesse

Fixes: 922ef8e7
("ir3: Make allocation of consts more generic and order independent")

Signed-off-by: Danylo Piliaiev <dpiliaiev@igalia.com>
Part-of: <!33161>