Enhance buffer load specialization pass to specialize past field extracts. (#8547)

This allows us to specialize functions whose argument is a sub element
of a constant buffer, instead of being only applicable to entire buffer
element. Closes #8421.

This change also implements a proper heuristic to determine when to
specialize the calls and defer the buffer loads.

This PR addresses a pathological case exposed in
`slangpy\slangpy\benchmarks\test_benchmark_tensor.py`, which used to
take 27ms to finish, and now takes 1.25ms.


For example, given:
```
struct Bottom
{
    float bigArray[1024];

    [mutating]
    void setVal(int index, float value) { bigArray[index] = value; }
}

struct Root
{
    Bottom top[2];
    [mutating]
    void setTopVal(int x, int y, float value)
    {
        top[x].setVal(y, value);
    }
}

RWStructuredBuffer<Root> sb;

[shader("compute")]
[numthreads(1, 1, 1)]
void compute_main(uint3 tid: SV_DispatchThreadID)
{
    sb[0].setTopVal(1, 2, 100.0f);
}
```

We are now able to specialize the call to `setTopVal` into:
```
void compute_main(uint3 tid: SV_DispatchThreadID)
{
    setTopVal_specialized(0, 1, 2, 100.0f);
}

void setTopVal_specialized(int sbIdx, int x, int y, float value)
{
      Bottom_setVal_specialized(sbIdx, x, y, value);
}

void Bottom_setVal_specialized(int sbIdx, int x, int y, float value)
{
     sb[sbIdx].top[x].bigArray[y] = value;
}
```

And get rid of all unnecessary loads. Achieving this requires a
combination of function call specialization and buffer-load-defer pass.
The buffer-load-defer pass has been completely rewritten to be more
correct and avoid introducing redundant loads.

This PR also adds tests to make sure pointers, bindless handles, and
loads from structured buffer or constant buffers works as expected.

2 files changed, 4 insertions, 4 deletions

diff --git a/prelude/slang-cpp-types.h b/prelude/slang-cpp-types.h
index 491438c80..26b45d53f 100644
--- a/prelude/slang-cpp-types.h
+++ b/prelude/slang-cpp-types.h
@@ -59,12 +59,12 @@ struct RWStructuredBuffer
 template<typename T>
 struct StructuredBuffer
 {
-    SLANG_FORCE_INLINE const T& operator[](size_t index) const
+    SLANG_FORCE_INLINE T& operator[](size_t index) const
     {
         SLANG_BOUND_CHECK(index, count);
         return data[index];
     }
-    const T& Load(size_t index) const
+    T& Load(size_t index) const
     {
         SLANG_BOUND_CHECK(index, count);
         return data[index];
diff --git a/prelude/slang-cuda-prelude.h b/prelude/slang-cuda-prelude.h
index 5c5335ac5..6c68cdb71 100644
--- a/prelude/slang-cuda-prelude.h
+++ b/prelude/slang-cuda-prelude.h
@@ -2312,7 +2312,7 @@ SLANG_FORCE_INLINE SLANG_CUDA_CALL uintptr_t UPTR_max(uintptr_t a, uintptr_t b)
 template<typename T>
 struct StructuredBuffer
 {
-    SLANG_CUDA_CALL const T& operator[](size_t index) const
+    SLANG_CUDA_CALL T& operator[](size_t index) const
     {
 #ifndef SLANG_CUDA_STRUCTURED_BUFFER_NO_COUNT
         SLANG_BOUND_CHECK(index, count);
@@ -2320,7 +2320,7 @@ struct StructuredBuffer
         return data[index];
     }
 
-    SLANG_CUDA_CALL const T& Load(size_t index) const
+    SLANG_CUDA_CALL T& Load(size_t index) const
     {
 #ifndef SLANG_CUDA_STRUCTURED_BUFFER_NO_COUNT
         SLANG_BOUND_CHECK(index, count);