Auto merge of #135753 - compiler-errors:from-ty-const, r=oli-obk

Get rid of `mir::Const::from_ty_const`

This function is strange, because it turns valtrees into `mir::Const::Value`, but the rest of the const variants stay as type system consts.

All of the callsites except for one in `instsimplify` (array length simplification of `ptr_metadata` call) just go through the valtree arm of the function, so it's easier to just create a `mir::Const` directly for those.

For the instsimplify case, if we have a type system const we should *keep* having a type system const, rather than turning it into a `mir::Const::Value`; it doesn't really matter in practice, though, bc `usize` has no padding, but it feels more principled.

compiler/rustc_mir_transform/src/instsimplify.rs

@@ -171,7 +171,7 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> {
         if let Rvalue::Len(ref place) = *rvalue {
             let place_ty = place.ty(self.local_decls, self.tcx).ty;
             if let ty::Array(_, len) = *place_ty.kind() {
-                let const_ = Const::from_ty_const(len, self.tcx.types.usize, self.tcx);
+                let const_ = Const::Ty(self.tcx.types.usize, len);
                 let constant = ConstOperand { span: DUMMY_SP, const_, user_ty: None };
                 *rvalue = Rvalue::Use(Operand::Constant(Box::new(constant)));
             }