Detect entrypoint more robustly when applying ConstRef hack during lowring

For shaders like tests/compute/simple.slang, which have a 'numthreads' attribute but no
'shader' attribute, the old compile request API would add an EntryPointAttribute to the
AST node of the entry point. However, the new API doesn't, and so a certain ConstRef hack
doesn't get applied when using the new API, leading to subsequent code generation issues.

This patch also checks for a 'numthreads' attribute when deciding whether to apply the
ConstRef hack.

This closes issue #6507 and helps to resolve issue #4760.

Author: aleino-nv

source/slang/slang-lower-to-ir.cpp

@@ -3214,7 +3214,8 @@ void collectParameterLists(
         // For now we will rely on a follow up pass to remove unnecessary temporary variables if
         // we can determine that they are never actually writtten to by the user.
         //
-        bool lowerVaryingInputAsConstRef = declRef.getDecl()->hasModifier<EntryPointAttribute>();
+        bool lowerVaryingInputAsConstRef = declRef.getDecl()->hasModifier<EntryPointAttribute>() ||
+                                           declRef.getDecl()->hasModifier<NumThreadsAttribute>();
 
         // Don't collect parameters from the outer scope if
         // we are in a `static` context.