add noreturn and unreachable

compiler/lower/lower.odin

@@ -716,6 +716,13 @@ lower_expr :: proc(state: ^State, expr_id: hir.Expr_Id) -> ir.Instruction_Id {
 					diagnostic=source.INVALID_DIAGNOSTIC,
 				})
 				_ = pop(&stack)
+			case .Unreachable:
+				last = append_instruction(state, ir.Instruction{
+					op=.Trap, span=expr.span, type=types.NORETURN, integer=1,
+					target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
+					diagnostic=source.INVALID_DIAGNOSTIC,
+				})
+				_ = pop(&stack)
 			case .String, .Array, .Struct, .Range, .Null, .Optional_Some, .Address, .Deref,
 			     .Index, .Slice, .Field, .Union_Tag, .Length, .Slice_Ptr, .Unwrap, .Orelse,
 			     .Try, .Catch, .Not, .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or:
@@ -1037,6 +1044,9 @@ lower_statements :: proc(state: ^State, statements: []hir.Stmt_Id) {
 				})
 			} else {
 				value := lower_expr(state, statement.expr)
+				if types.is_noreturn(state.hir_module.exprs[statement.expr].type) {
+					continue
+				}
 				append_instruction(state, ir.Instruction{
 					op=.Return,
 					span=statement.span,
@@ -1680,11 +1690,22 @@ lower_body :: proc(hir_module: ^hir.Module, function: hir.Function, allocator: m
 	}
 
 	lower_statements(&state, function.body)
-	if len(state.instructions) == 0 ||
-	   (state.instructions[len(state.instructions)-1].op != .Return &&
-	    state.instructions[len(state.instructions)-1].op != .Return_Void) {
+	last_terminates := false
+	if len(state.instructions) > 0 {
+		last_instruction := state.instructions[len(state.instructions)-1]
+		last_terminates = last_instruction.op == .Return || last_instruction.op == .Return_Void ||
+		                  last_instruction.op == .Trap ||
+		                  last_instruction.op == .Call && types.is_noreturn(last_instruction.type)
+	}
+	if !last_terminates {
 		if types.is_void(function.result) {
 			append_instruction(&state, ir.Instruction{op=.Return_Void, type=types.VOID, target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION, diagnostic=source.INVALID_DIAGNOSTIC})
+		} else if types.is_noreturn(function.result) {
+			append_instruction(&state, ir.Instruction{
+				op=.Trap, type=types.NORETURN, integer=1,
+				target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
+				diagnostic=source.INVALID_DIAGNOSTIC,
+			})
 		} else {
 			value := append_instruction(&state, ir.Instruction{
 				op=.Const,