compiler: fix buglet in lshift type determination

    
    It's possible to construct an lshift expression using unsafe.Sizeof
    that is technically a compile-time constant but can't be evaluated
    without going through backend methods. In this case, insure that
    Type::make_non_abstract_type is called on the numeric operand of the
    shift (as opposed to leaving as abstract), to avoid an assert later on
    in the compiler flow.
    
    Fixes golang/go#21372.
    
    Reviewed-on: https://go-review.googlesource.com/54370

From-SVN: r251006

gcc/go/gofrontend/MERGE

-5fd112e5c2968e94761c41519c451d789e23a92b
+480fdfa9dd416bd17115a94fa6021c4dd805fc39
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -5655,7 +5655,7 @@ Binary_expression::do_determine_type(const Type_context* context)
 
   Type_context subcontext(*context);
 
-  if (is_constant_expr)
+  if (is_constant_expr && !is_shift_op)
     {
       subcontext.type = NULL;
       subcontext.may_be_abstract = true;