vkd3d-shader/hlsl: Introduce the "error" type.

Currently, if an expression successfully parses according to the bison grammar,
but for one reason or another cannot generate a meaningful IR instruction, we
abort parsing with YYABORT. This includes, for example, an undefined variable or
function, invalid swizzle or field reference, or a constructor with a complex or
non-numeric data type.

Aborting parsing is unfortunate, however, because it means that any further
errors in the program cannot be caught by the programmer, increasing the number
of times they will need to fix errors and recompile.

The idea of this patch is that any such expression will instead generate an IR
node whose data type is of HLSL_CLASS_ERROR. Any further expression which would
consume an "error" typed instruction will instead immediately return an
expression of type "error" (probably the same one) instead of aborting or doing
any other type-checking.

Currently these "error" instructions should not pass the parsing stage, since
hlsl_compile_shader() will immediately notice that compilation has failed and
skip any optimization, lowering, or bytecode-writing.

A further direction to take this is to pre-allocate one "error" expression
immediately when creating the HLSL parser, and return that expression when we
fail to allocate an hlsl_ir_node of any type. This means we do not need to
handle allocation errors when constructing nodes, saving us quite a lot of error
handling (which is not only tedious but currently often broken, if nothing else
by virtue of neglecting cleanup of local variables).

libs/vkd3d-shader/hlsl_codegen.c

@@ -1661,6 +1661,7 @@ static bool copy_propagation_transform_load(struct hlsl_ctx *ctx,
              * solve that problem for us, since we may be called on a partial
              * block, but DCE deletes dead stores, so it needs to be able to
              * see the whole program. */
+        case HLSL_CLASS_ERROR:
             return false;
 
         case HLSL_CLASS_CONSTANT_BUFFER: