don't ignore sign for ABI check

Author: RalfJung

tests/pass/function_calls/abi_compat.rs

@@ -1,7 +1,5 @@
-#![feature(portable_simd)]
 use std::mem;
 use std::num;
-use std::simd;
 
 #[derive(Copy, Clone)]
 struct Zst;
@@ -56,19 +54,15 @@ fn test_abi_newtype<T: Copy>(t: T) {
 
 fn main() {
     // Here we check:
-    // - unsigned vs signed integer is allowed
-    // - u32/i32 vs char is allowed
+    // - u32 vs char is allowed
     // - u32 vs NonZeroU32/Option<NonZeroU32> is allowed
     // - reference vs raw pointer is allowed
     // - references to things of the same size and alignment are allowed
     // These are very basic tests that should work on all ABIs. However it is not clear that any of
     // these would be stably guaranteed. Code that relies on this is equivalent to code that relies
     // on the layout of `repr(Rust)` types. They are also fragile: the same mismatches in the fields
     // of a struct (even with `repr(C)`) will not always be accepted by Miri.
-    test_abi_compat(0u32, 0i32);
-    test_abi_compat(simd::u32x8::splat(1), simd::i32x8::splat(1));
     test_abi_compat(0u32, 'x');
-    test_abi_compat(0i32, 'x');
     test_abi_compat(42u32, num::NonZeroU32::new(1).unwrap());
     test_abi_compat(0u32, Some(num::NonZeroU32::new(1).unwrap()));
     test_abi_compat(&0u32, &0u32 as *const u32);