NRVO: Allow occurrences of the return place in var debug info

The non-use occurrence of the return place in var debug info does not
currently inhibit NRVO optimization, but it will fail assertion in
`visit_place` when optimization is performed.

Relax assertion check to allow the return place in var debug info.

This case might be impossible to hit in optimization pipelines as of
now, but can be encountered in customized mir-opt-level=2 pipeline with
copy propagation disabled. For example in:

```
pub fn b(s: String) -> String {
    a(s)
}

#[inline]
pub fn a(s: String) -> String {
    let x = s;
    let y = x;
    y
}
```

compiler/rustc_mir/src/transform/nrvo.rs

@@ -1,6 +1,6 @@
 use rustc_hir::Mutability;
 use rustc_index::bit_set::HybridBitSet;
-use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
+use rustc_middle::mir::visit::{MutVisitor, NonUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::{self, BasicBlock, Local, Location};
 use rustc_middle::ty::TyCtxt;
 
@@ -196,9 +196,10 @@ impl MutVisitor<'tcx> for RenameToReturnPlace<'tcx> {
         self.super_terminator(terminator, loc);
     }
 
-    fn visit_local(&mut self, l: &mut Local, _: PlaceContext, _: Location) {
-        assert_ne!(*l, mir::RETURN_PLACE);
-        if *l == self.to_rename {
+    fn visit_local(&mut self, l: &mut Local, ctxt: PlaceContext, _: Location) {
+        if *l == mir::RETURN_PLACE {
+            assert_eq!(ctxt, PlaceContext::NonUse(NonUseContext::VarDebugInfo));
+        } else if *l == self.to_rename {
             *l = mir::RETURN_PLACE;
         }
     }