Use START_BLOCK in codegen_naked_asm

Author: bjorn3

compiler/rustc_codegen_ssa/src/mir/naked_asm.rs

@@ -1,6 +1,6 @@
 use rustc_attr_parsing::InstructionSetAttr;
 use rustc_middle::mir::mono::{Linkage, MonoItem, MonoItemData, Visibility};
-use rustc_middle::mir::{Body, InlineAsmOperand};
+use rustc_middle::mir::{Body, InlineAsmOperand, START_BLOCK};
 use rustc_middle::ty::layout::{HasTyCtxt, HasTypingEnv, LayoutOf};
 use rustc_middle::ty::{Instance, TyCtxt};
 use rustc_middle::{bug, ty};
@@ -22,7 +22,7 @@ pub(crate) fn codegen_naked_asm<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         line_spans,
         targets: _,
         unwind: _,
-    } = mir.basic_blocks.iter().next().unwrap().terminator().kind
+    } = mir.basic_blocks[START_BLOCK].terminator().kind
     else {
         bug!("#[naked] functions should always terminate with an asm! block")
     };