rs6000: Backport u8bit_cint_operand predicate

Backport from master.

2020-05-11  Kelvin Nilsen  <wschmidt@linux.ibm.com>

gcc/
	* config/rs6000/predicates.md (u8bit_cint_operand): New predicate.

gcc/config/rs6000/predicates.md

@@ -234,6 +234,11 @@
   (and (match_code "const_int")
        (match_test "IN_RANGE (INTVAL (op), 0, 127)")))
 
+;; Return 1 if op is an unsigned 8-bit constant integer.
+(define_predicate "u8bit_cint_operand"
+  (and (match_code "const_int")
+       (match_test "IN_RANGE (INTVAL (op), 0, 255)")))
+
 ;; Return 1 if op is a signed 8-bit constant integer.
 ;; Integer multiplication complete more quickly
 (define_predicate "s8bit_cint_operand"