core: Allocate exceptions at the beginning of function

Only one instance of exception is necessary, as exceptions will always
be initialized before being thrown.

nac3core/src/codegen/expr.rs

@@ -459,9 +459,14 @@ impl<'ctx, 'a> CodeGenContext<'ctx, 'a> {
         params: [Option<IntValue<'ctx>>; 3],
         loc: Location,
     ) {
+        let zelf = if let Some(exception_val) = self.exception_val {
+            exception_val
+        } else {
             let ty = self.get_llvm_type(generator, self.primitives.exception).into_pointer_type();
             let zelf_ty: BasicTypeEnum = ty.get_element_type().into_struct_type().into();
             let zelf = generator.gen_var_alloc(self, zelf_ty, Some("exn")).unwrap();
+            self.exception_val.insert(zelf).to_owned()
+        };
         let int32 = self.ctx.i32_type();
         let zero = int32.const_zero();
         unsafe {

nac3core/src/codegen/mod.rs

@@ -146,6 +146,7 @@ pub struct CodeGenContext<'ctx, 'a> {
     pub const_strings: HashMap<String, BasicValueEnum<'ctx>>,
     // stores the alloca for variables
     pub init_bb: BasicBlock<'ctx>,
+    pub exception_val: Option<PointerValue<'ctx>>,
     /// The header and exit basic blocks of a loop in this context. See
     /// https://llvm.org/docs/LoopTerminology.html for explanation of these terminology.
     pub loop_target: Option<(BasicBlock<'ctx>, BasicBlock<'ctx>)>,
@@ -802,6 +803,7 @@ pub fn gen_func_impl<'ctx, G: CodeGenerator, F: FnOnce(&mut G, &mut CodeGenConte
         type_cache,
         primitives,
         init_bb,
+        exception_val: Default::default(),
         builder,
         module,
         unifier,