fix compiler bug related to `#%variable-reference' on local

The first compiler pass didn't properly shift the stack offset
when adjusting the context of a varref of a local.

Closes PR 12258

collects/tests/racket/optimize.rktl

@@ -1643,6 +1643,17 @@
                (regexp-match #rx"1e[+]?100" (exn-message exn))))
 (test (inexact->exact 1e100) (lambda (x) (inexact->exact (fl/ (fl- x 0.0) 1.0))) 1e100)
 
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Check that compiler handles shifting `#%variable-reference'
+
+(test #f
+      'varref-shift
+      (let ()
+        (define (f #:x [x #f]) #f)
+        (define (g #:y [y #f])
+          (begin (f) #f))
+        #f))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (report-errs)

src/racket/src/compile.c

@@ -1745,7 +1745,6 @@ static Scheme_Object *shift_compiled_expression(Scheme_Object *v, int delta, int
   switch (SCHEME_TYPE(v)) {
   case scheme_compiled_toplevel_type:
   case scheme_compiled_quote_syntax_type:
-  case scheme_varref_form_type:
     return v;
   case scheme_local_type:
     {
@@ -1905,6 +1904,18 @@ static Scheme_Object *shift_compiled_expression(Scheme_Object *v, int delta, int
 
       return (Scheme_Object *)lh;
     }
+  case scheme_varref_form_type:
+    {
+      Scheme_Object *sv;
+
+      sv = shift_compiled_expression(SCHEME_PTR1_VAL(v), delta, skip);
+      SCHEME_PTR1_VAL(v) = sv;
+      
+      sv = shift_compiled_expression(SCHEME_PTR2_VAL(v), delta, skip);
+      SCHEME_PTR2_VAL(v) = sv;
+      
+      return v;
+    }
   default:
     scheme_signal_error("internal error: compile-time shift failed: %d", SCHEME_TYPE(v));
     return NULL;

src/racket/src/optimize.c

@@ -223,6 +223,7 @@ int scheme_omittable_expr(Scheme_Object *o, int vals, int fuel, int resolved,
       || (vtype == scheme_case_lambda_sequence_type)
       || (vtype == scheme_case_lambda_sequence_type)
       || (vtype == scheme_quote_syntax_type)
+      || (vtype == scheme_varref_form_type)
       || (vtype == scheme_compiled_quote_syntax_type)) {
     note_match(1, vals, warn_info);
     return ((vals == 1) || (vals < 0));