[IR] Allow a maximum of 12 SNode indices (#6901)

Fixes #6900

Author: AmesingFlank

docs/lang/articles/internals/internal.md

@@ -343,5 +343,5 @@ assert mapping_b == {0: 1, 1: 0}
 # the virtual first index exposed to the user comes second in memory layout.
 ```
 
-Taichi supports up to 8 (`constexpr int taichi_max_num_indices = 8`)
+Taichi supports up to 12 (`constexpr int taichi_max_num_indices = 12`)
 virtual indices and physical indices.

taichi/inc/constants.h

@@ -2,7 +2,7 @@
 
 #include <cstddef>
 
-constexpr int taichi_max_num_indices = 8;
+constexpr int taichi_max_num_indices = 12;
 // legacy: only used in cc and opengl backends
 constexpr int taichi_max_num_args = 8;
 // used in llvm backend: only the first 32 arguments can be types.ndarray

taichi/runtime/metal/shaders/runtime_structs.metal.h

@@ -17,7 +17,7 @@
 
 #include "taichi/inc/constants.h"
 
-static_assert(taichi_max_num_indices == 8,
+static_assert(taichi_max_num_indices == 12,
               "Please update kTaichiMaxNumIndices");
 static_assert(sizeof(char *) == 8, "Metal pointers are 64-bit.");
 #define METAL_BEGIN_RUNTIME_STRUCTS_DEF
@@ -28,7 +28,7 @@ static_assert(sizeof(char *) == 8, "Metal pointers are 64-bit.");
 // clang-format off
 METAL_BEGIN_RUNTIME_STRUCTS_DEF
 STR(
-    constant constexpr int kTaichiMaxNumIndices = 8;
+    constant constexpr int kTaichiMaxNumIndices = 12;
     constant constexpr int kTaichiNumChunks = 1024;
     constant constexpr int kAlignment = 8;
     using PtrOffset = int32_t;