Make simd_extract panic at runtime on non-const index again

This is necessary to compile packed_simd

src/intrinsics/simd.rs

@@ -146,10 +146,17 @@ pub(super) fn codegen_simd_intrinsic_call<'tcx>(
             let idx_const = if let Some(idx_const) = crate::constant::mir_operand_get_const_val(fx, idx) {
                 idx_const
             } else {
-                fx.tcx.sess.span_fatal(
+                fx.tcx.sess.span_warn(
                     span,
                     "Index argument for `simd_extract` is not a constant",
                 );
+                let res = crate::trap::trap_unimplemented_ret_value(
+                    fx,
+                    ret.layout(),
+                    "Index argument for `simd_extract` is not a constant",
+                );
+                ret.write_cvalue(fx, res);
+                return;
             };
 
             let idx = idx_const.val.try_to_bits(Size::from_bytes(4 /* u32*/)).unwrap_or_else(|| panic!("kind not scalar: {:?}", idx_const));

src/trap.rs

@@ -67,3 +67,15 @@ pub(crate) fn trap_unimplemented(fx: &mut FunctionCx<'_, '_, impl Module>, msg:
     let true_ = fx.bcx.ins().iconst(types::I32, 1);
     fx.bcx.ins().trapnz(true_, TrapCode::User(!0));
 }
+
+/// Like `trap_unimplemented` but returns a fake value of the specified type.
+///
+/// Trap code: user65535
+pub(crate) fn trap_unimplemented_ret_value<'tcx>(
+    fx: &mut FunctionCx<'_, 'tcx, impl Module>,
+    dest_layout: TyAndLayout<'tcx>,
+    msg: impl AsRef<str>,
+) -> CValue<'tcx> {
+    trap_unimplemented(fx, msg);
+    CValue::by_ref(Pointer::const_addr(fx, 0), dest_layout)
+}