Rollup merge of #136180 - lukas-code:typed-valtree, r=oli-obk

Introduce a wrapper for "typed valtrees" and properly check the type before extracting the value

This PR adds a new wrapper type `ty::Value` to replace the tuple `(Ty, ty::ValTree)` and become the new canonical representation of type-level constant values.

The value extraction methods `try_to_bits`/`try_to_bool`/`try_to_target_usize` are moved to this new type. For `try_to_bits` in particular, this avoids some redundant matches on `ty::ConstKind::Value`. Furthermore, these methods and will now properly check the type before extracting the value, which fixes some ICEs.

The name `ty::Value` was chosen to be consistent with `ty::Expr`.

Commit 1 should be non-functional and commit 2 adds the type check.

---

fixes rust-lang/rust#131102
supercedes rust-lang/rust#136130

r? `@oli-obk`
cc `@FedericoBruzzone` `@BoxyUwU`

Author: matthiaskrgr

src/intrinsics/simd.rs

@@ -640,7 +640,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 let (dest, dest_len) = this.project_to_simd(dest)?;
 
                 let index =
-                    generic_args[2].expect_const().try_to_valtree().unwrap().0.unwrap_branch();
+                    generic_args[2].expect_const().to_value().valtree.unwrap_branch();
                 let index_len = index.len();
 
                 assert_eq!(left_len, right_len);