lazy analysis of top level declarations

previously, we had lazy analysis of top level declarations,
but if a declaration was referenced within a compile-time
if or switch statement, that would still add the top
level declaration to the resolution queue.

now we have a declref ir instruction, which is only resolved
if we analyze the instruction. this takes into account comptime
branching.

closes #270

src/codegen.cpp

@@ -2519,6 +2519,7 @@ static LLVMValueRef ir_render_instruction(CodeGen *g, IrExecutable *executable,
         case IrInstructionIdCanImplicitCast:
         case IrInstructionIdSetGlobalAlign:
         case IrInstructionIdSetGlobalSection:
+        case IrInstructionIdDeclRef:
             zig_unreachable();
         case IrInstructionIdReturn:
             return ir_render_return(g, executable, (IrInstructionReturn *)instruction);