Rework OperandRef::extract_field to stop calling to_immediate_scalar on things which are already immediates

That means it stops trying to truncate things that are already `i1`s.

tests/crashes/project-to-simd-array-field.rs

@@ -0,0 +1,33 @@
+//@ known-bug: #137108
+//@compile-flags: -Copt-level=3
+
+// If you fix this, put it in the corresponding codegen test,
+// not in a UI test like the readme says.
+
+#![crate_type = "lib"]
+
+#![feature(repr_simd, core_intrinsics)]
+
+#[allow(non_camel_case_types)]
+#[derive(Clone, Copy)]
+#[repr(simd)]
+struct i32x3([i32; 3]);
+
+const _: () = { assert!(size_of::<i32x3>() == 16) };
+
+#[inline(always)]
+fn to_array3(a: i32x3) -> [i32; 3] {
+    a.0
+}
+
+// CHECK-LABEL: simd_add_self_then_return_array_packed(
+// CHECK-SAME: ptr{{.+}}sret{{.+}}%[[RET:.+]],
+// CHECK-SAME: ptr{{.+}}%a)
+#[no_mangle]
+pub fn simd_add_self_then_return_array_packed(a: i32x3) -> [i32; 3] {
+    // CHECK: %[[T1:.+]] = load <3 x i32>, ptr %a
+    // CHECK: %[[T2:.+]] = shl <3 x i32> %[[T1]], <i32 1, i32 1, i32 1>
+    // CHECK: store <3 x i32> %[[T2]], ptr %[[RET]]
+    let b = unsafe { core::intrinsics::simd::simd_add(a, a) };
+    to_array3(b)
+}