fix zero_extract lshiftrt insv combiner pattern bug

From-SVN: r14044

gcc/config/rs6000/rs6000.md

@@ -2138,7 +2138,7 @@
 			 (match_operand:SI 2 "const_int_operand" "i"))
 	(lshiftrt:SI (match_operand:SI 3 "gpc_reg_operand" "r")
 		     (match_operand:SI 4 "const_int_operand" "i")))]
-  ""
+  "(32 - INTVAL (operands[4])) >= INTVAL (operands[1])"
   "*
 {
   int shift = INTVAL (operands[4]) & 31;