move ConstValue into mir

this way we have mir::ConstValue and ty::ValTree as reasonably parallel

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -6,8 +6,8 @@ use crate::glue;
 use crate::traits::*;
 use crate::MemFlags;
 
-use rustc_middle::mir;
-use rustc_middle::mir::interpret::{alloc_range, ConstValue, Pointer, Scalar};
+use rustc_middle::mir::interpret::{alloc_range, Pointer, Scalar};
+use rustc_middle::mir::{self, ConstValue};
 use rustc_middle::ty::layout::{LayoutOf, TyAndLayout};
 use rustc_middle::ty::Ty;
 use rustc_target::abi::{self, Abi, Align, Size};
@@ -86,7 +86,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
 
     pub fn from_const<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
         bx: &mut Bx,
-        val: ConstValue<'tcx>,
+        val: mir::ConstValue<'tcx>,
         ty: Ty<'tcx>,
     ) -> Self {
         let layout = bx.layout_of(ty);