IR: fixes for subscript accessors (#322)

* IR: fixes for subscript accessors

Fixes #320

This is a bunch of fixes for handling of `__subscript` operations on builtin types (notably `RWStructuredBuffer` and `StructuredBuffer` at this point).

- Automatically add a `GetterDecl` to any subscript decalratio was declithout any accessors. This avoids hitting a null- dereference in the emit logic.

- Add a notion of a `RefAccessor` (declared with `ref`) as a peer to getters and setters. The idea is that a `ref` accessor returns a pointer to the element data, so that it can be used for both getting and setting values. This is closer to the behavior of `RWStructuredBuffer` element access in HLSL.

- Fixes for dealing with "access chains" where there might be a combination of a subscript (where the is a `get` and `set` but no `ref`) and member access, so that we have to read the base value into a temp, modify it, and then write it back.
  - This logic is still a bit of a mess, so we will eventually want to take a more consistent pass over this to deal with how we "materialize" values for setters.

- Update `RWStructuredBuffer` to have a `ref` accessor, and then fix up the IR tests to handle the new opcode that I added for it.
  - Note: I didn't handle this as an intrinsic simply because the `tests/ir/*` tests aren't really set up to handle builtins with ugly mangled names.

* Fixup: type error in VM for buffer element ref

I was using the result type of the op as the element type for computing the element address, but the result type is a pointer to the real element type.
This caused test failures on 64-bit platforms, where the stride of the buffer in the `ir/factorial` test needs to be 4.

The fix is to assume the result type is a pointer, and extract the pointed-to type out of that.

1 files changed, 4 insertions, 3 deletions

diff --git a/tests/ir/loop.slang.expected b/tests/ir/loop.slang.expected
index 720f80091..ab7ad687c 100644
--- a/tests/ir/loop.slang.expected
+++ b/tests/ir/loop.slang.expected
@@ -73,9 +73,10 @@ block %16:
 	call(@_S031GroupMemoryBarrierWithGroupSyncp0pV)
 	let  %40	: RWStructuredBuffer<vector<float,4>>	= load(@_SV06output)
 	let  %41	: uint	= load(%5)
-	let  %42	: Ptr<vector<float,4>>	= getElementPtr(@_SV01s, 0)
-	let  %43	: vector<float,4>	= load(%42)
-	bufferStore(%40, %41, %43)
+	let  %42	: Ptr<vector<float,4>>	= bufferElementRef(%40, %41)
+	let  %43	: Ptr<vector<float,4>>	= getElementPtr(@_SV01s, 0)
+	let  %44	: vector<float,4>	= load(%43)
+	store(%42, %44)
 	return_void()
 }