rs6000: Call library for block memory compare when optimizing for size

gcc/
	* config/rs6000/rs6000.md (cmpmemsi): Fail when optimizing for size.

gcc/testsuite/
	* gcc.target/powerpc/block-cmp-3.c: New.

gcc/config/rs6000/rs6000.md

@@ -10146,6 +10146,9 @@
 	      (use (match_operand:SI 4))])]
   "TARGET_POPCNTD"
 {
+  if (optimize_insn_for_size_p ())
+    FAIL;
+
   if (expand_block_compare (operands))
     DONE;
   else

gcc/testsuite/gcc.target/powerpc/block-cmp-3.c

@@ -0,0 +1,8 @@
+/* { dg-do compile } */
+/* { dg-options "-Os" } */
+/* { dg-final { scan-assembler-times {\mb[l]? memcmp\M} 1 } }  */
+
+int foo (const char* s1, const char* s2)
+{
+  return __builtin_memcmp (s1, s2, 4);
+}