gcc.old: PR port-vax/57646 patch provided by Kalvis Duckmanton [5/21]

Shift right by positive values that are less than HOST_BITS_PER_WIDE_INT
to avoid illegal instruction exceptions on VAX.

XXXRO: Hidden within ``#ifdef NB_FIX_VAX_BACKEND'' and enabled only for
vax at the moment.

external/gpl3/gcc.old/dist/gcc/dse.c

@@ -301,7 +301,12 @@ static unsigned HOST_WIDE_INT
 lowpart_bitmask (int n)
 {
   unsigned HOST_WIDE_INT mask = HOST_WIDE_INT_M1U;
-#if 1 // XXXMRG
+#ifdef NB_FIX_VAX_BACKEND
+  if (n < 1)
+    return 0;
+  if (n >= HOST_BITS_PER_WIDE_INT)
+    return mask;
+#else // XXXMRG
   gcc_assert(n >= 0 && n <= HOST_BITS_PER_WIDE_INT);
   if (n == 0)
     return 0;
@@ -1340,6 +1345,10 @@ all_positions_needed_p (store_info *s_info, poly_int64 start,
 	  return false;
       return true;
     }
+#ifdef NB_FIX_VAX_BACKEND
+  if (const_start >= HOST_BITS_PER_WIDE_INT || const_start < 0)
+    return true;
+#endif
   else
     {
       unsigned HOST_WIDE_INT mask