Fix project_deref() implementation

I'm not really sure what is wrong here, but I was getting load
type mismatches in the debuginfo code (which is the only place
using this function).

Replacing the project_deref() implementation with a generic
load_operand + deref did the trick.

compiler/rustc_codegen_ssa/src/mir/debuginfo.rs

@@ -274,7 +274,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 match *elem {
                     mir::ProjectionElem::Deref => {
                         indirect_offsets.push(Size::ZERO);
-                        place = place.project_deref(bx);
+                        place = bx.load_operand(place).deref(bx.cx());
                     }
                     mir::ProjectionElem::Field(field, _) => {
                         let i = field.index();