nir: Add scoped_memory_barrier intrinsic

Add a NIR instrinsic that represent a memory barrier in SPIR-V /
Vulkan Memory Model, with extra attributes that describe the barrier:

- Ordering: whether is an Acquire or Release;
- "Cache control": availability ("ensure this gets written in the memory")
  and visibility ("ensure my cache is up to date when I'm reading");
- Variable modes: which memory types this barrier applies to;
- Scope: how far this barrier applies.

Note that unlike in SPIR-V, the "Storage Semantics" and the "Memory
Semantics" are split into two different attributes so we can use
variable modes for the former.

NIR passes that took barriers in consideration were also changed

- nir_opt_copy_prop_vars: clean up the values for the mode of an
  ACQUIRE barrier.  Copy propagation effect is to "pull up a load" (by
  not performing it), which is what ACQUIRE restricts.

- nir_opt_dead_write_vars and nir_opt_combine_writes: clean up the
  pending writes for the modes of an RELEASE barrier.  Dead writes
  effect is to "push down a store", which is what RELEASE restricts.

- nir_opt_access: treat the ACQUIRE and RELEASE as a full barrier for
  the modes.  This is conservative, but since this is a GL-specific
  pass, doesn't make a difference for now.

v2: Fix the scoped barrier handling in copy propagation.  (Jason)
    Add scoped barrier handling to nir_opt_access and
    nir_opt_combine_writes.  (Rhys)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>

src/compiler/nir/nir_intrinsics.py

@@ -126,6 +126,12 @@ TYPE = "NIR_INTRINSIC_TYPE"
 SWIZZLE_MASK = "NIR_INTRINSIC_SWIZZLE_MASK"
 # Driver location of attribute
 DRIVER_LOCATION = "NIR_INTRINSIC_DRIVER_LOCATION"
+# Ordering and visibility of a memory operation
+MEMORY_SEMANTICS = "NIR_INTRINSIC_MEMORY_SEMANTICS"
+# Modes affected by a memory operation
+MEMORY_MODES = "NIR_INTRINSIC_MEMORY_MODES"
+# Scope of a memory operation
+MEMORY_SCOPE = "NIR_INTRINSIC_MEMORY_SCOPE"
 
 #
 # Possible flags:
@@ -206,6 +212,12 @@ intrinsic("is_helper_invocation", dest_comp=1, flags=[CAN_ELIMINATE])
 # intrinsic.
 barrier("memory_barrier")
 
+# Memory barrier with explicit scope.  Follows the semantics of SPIR-V
+# OpMemoryBarrier, used to implement Vulkan Memory Model.  Storage that the
+# barrierr applies is represented using NIR variable modes.
+intrinsic("scoped_memory_barrier",
+          indices=[MEMORY_SEMANTICS, MEMORY_MODES, MEMORY_SCOPE])
+
 # Shader clock intrinsic with semantics analogous to the clock2x32ARB()
 # GLSL intrinsic.
 # The latter can be used as code motion barrier, which is currently not