Rollup merge of #126157 - RalfJung:scalar-i128, r=compiler-errors

add missing Scalar::from_i128

We seem to have `from` methods for all primitive types except `i128`... let's complete the set of methods. (This came up in https://github.com/rust-lang/miri/pull/3622.)

compiler/rustc_middle/src/mir/interpret/value.rs

@@ -196,6 +196,11 @@ impl<Prov> Scalar<Prov> {
         Self::from_int(i, Size::from_bits(64))
     }
 
+    #[inline]
+    pub fn from_i128(i: i128) -> Self {
+        Self::from_int(i, Size::from_bits(128))
+    }
+
     #[inline]
     pub fn from_target_isize(i: i64, cx: &impl HasDataLayout) -> Self {
         Self::from_int(i, cx.data_layout().pointer_size)