Rollup merge of #58667 - kenta7777:reduce-mir-code-repetition, r=petrochenkov

Reduce Miri-related Code Repetition `like (n << amt) >> amt`

This Pull Request fixes a part of [#49937](https://github.com/rust-lang/rust/issues/49937).

src/librustc_mir/interpret/place.rs

@@ -7,6 +7,7 @@
 
 use rustc::hir;
 use rustc::mir;
+use rustc::mir::interpret::truncate;
 use rustc::ty::{self, Ty};
 use rustc::ty::layout::{self, Size, Align, LayoutOf, TyLayout, HasDataLayout, VariantIdx};
 use rustc::ty::TypeFoldable;
@@ -965,8 +966,7 @@ pub fn write_discriminant_index(
                 // their computation, but the in-memory tag is the smallest possible
                 // representation
                 let size = tag.value.size(self);
-                let shift = 128 - size.bits();
-                let discr_val = (discr_val << shift) >> shift;
+                let discr_val = truncate(discr_val, size);
 
                 let discr_dest = self.place_field(dest, 0)?;
                 self.write_scalar(Scalar::from_uint(discr_val, size), discr_dest)?;