Basic IR support for static const globals (shader-slang#404)

* Basic IR support for `static const` globals

Our strategy for lowering global *variables* can fall back to putting their initialization into a function, but that isn't really appropriate for global constants (it also isn't appropriate for arrays, but we'll need to deal with that seaprately).

This change adds a distinct case for global constants (rather than treating them as variables), and forces the emission logic to always emit them as a single expression.
Doing this makes assumptions about how the IR for these constants gets emitted (and what optimziations might do to it).
In order to make things work, I had to switch the handling of initializer-list expressions to not be lowered via temporaries and mutation (since that isn't a good fit for reverting to a single expression).

I've added a single test case to ensure that this works in the simplest scenario. My next priority will be to see if this unblocks my work in Falcor.

* Fixup: bug fixes

Author: Tim Foley

source/slang/bytecode.cpp

@@ -918,12 +918,15 @@ BytecodeGenerationPtr<BCSymbol> generateBytecodeSymbolForInst(
         break;
 
     case kIROp_global_var:
+    case kIROp_global_constant:
         {
             auto bcVar = allocate<BCSymbol>(context);
 
             bcVar->op = inst->op;
             bcVar->typeID = getTypeID(context, inst->type);
 
+            // TODO: actually need to intialize with body instructions
+
             return bcVar;
         }
         break;