nir: add many passes that lower and optimize 16-bit input/outputs and samplers

Added:
* a pass that renumbers bases of IO intrinsics
* a pass that converts mediump IO to 16 bits, optionally using the new
  packed varying slots
* a pass that sets (forces) mediump in IO intrinsics (for testing)
* a pass that remaps VARYING_SLOT_VAR[0..15]_16BIT to VARYING_SLOT_VAR[0..31]
  (if some shader stages don't want packed varyings)
* a pass that folds type conversions around texture opcodes into those
  opcodes (e.g. tex(f2f32(coord), ..) is changed into tex accepting f16)
* a pass that changes (legalizes) sampler src and dst types based on specified
  hw constraints (e.g. derivatives must be the same type as coordinates)

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9050>

src/compiler/nir/meson.build

@@ -168,7 +168,7 @@ files_libnir = files(
   'nir_lower_io_to_scalar.c',
   'nir_lower_io_to_vector.c',
   'nir_lower_multiview.c',
-  'nir_lower_mediump_outputs.c',
+  'nir_lower_mediump.c',
   'nir_lower_memcpy.c',
   'nir_lower_memory_model.c',
   'nir_lower_non_uniform_access.c',