[ir] Represent fields more direclty

Previously, a `StructType` was an ordinary instruction that took a variable number of types are operands, representing the types of fields.
This ends up being inconvenient for a few reasons:

- To add decorations to the fields, you'd end up having to decorate the struct type instead (SPIR-V has this problem)

- You need to compute field indices during lowering, when you might prefer to defer that until later

- The get/set field operations now need an index, which needs to be an explicit operand, which means a magic numeric literal floating around to represent the index

The new approach fixes for the first two of these, and at least makes the last one a bit nicer.
A `StructDecl` is now a parent instruction, and its sub-instructions represent the fields of the type - each field is an explicit instruction of type `StructField`.
The operation to extract a field takes a direct reference the struct field, so everything is quite explicit.

1 files changed, 4 insertions, 3 deletions

diff --git a/source/slang/ir-inst-defs.h b/source/slang/ir-inst-defs.h
index 8445e5a1e..d594d2755 100644
--- a/source/slang/ir-inst-defs.h
+++ b/source/slang/ir-inst-defs.h
@@ -14,7 +14,7 @@ INST(BoolType,    type.bool,      0, 0)
 INST(Float32Type, type.f32,       0, 0)
 INST(Int32Type,   type.i32,       0, 0)
 INST(UInt32Type,  type.u32,       0, 0)
-INST(StructType,  type.struct,    0, 0)
+INST(StructType,  type.struct,    0, PARENT)
 INST(FuncType,    func_type,      0, 0)
 
 INST(IntLit,      integer_constant,   0, 0)
@@ -26,9 +26,10 @@ INST(Module,      module, 0, PARENT)
 INST(Func,        func,   0, PARENT)
 INST(Block,       block,  0, PARENT)
 
-INST(Param,           param,  0, 0)
+INST(Param,         param,  0, 0)
+INST(StructField,   field,  0, 0)
 
-INST(FieldExtract,    get_field,      1, 0)
+INST(FieldExtract,    get_field,      2, 0)
 INST(ReturnVal,       return_val,     1, 0)
 INST(ReturnVoid, return_void, 1, 0)