match.pd: Relax rule to include POLY_INT_CSTs

match.pd has a rule to simplify an extension, operation and truncation
back to the original type:

 (simplify
   (convert (op:s@0 (convert1?@3 @1) (convert2?@4 @2)))

Currently it handles cases in which @2 is an INTEGER_CST, but it
also works for POLY_INT_CSTs.[*]

For INTEGER_CST it doesn't matter whether we test @2 or @4,
but for POLY_INT_CST it is possible to have unfolded (convert …)s.

Originally I saw this leading to some bad ivopts decisions, because
we weren't folding away redundancies from candidate iv expressions.
It's also possible to test the fold directly using the SVE ACLE.

[*] Not all INTEGER_CST rules work for POLY_INT_CSTs, since extensions
    don't necessarily distribute over the internals of the POLY_INT_CST.
    But in this case that isn't an issue.

gcc/
	* match.pd: Simplify an extend-operate-truncate sequence involving
	a POLY_INT_CST.

gcc/testsuite/
	* gcc.target/aarch64/sve/acle/general/cntb_1.c: New test.

gcc/match.pd

@@ -6175,7 +6175,7 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
 	&& (types_match (@1, @2)
 	    /* Or the second operand is const integer or converted const
 	       integer from valueize.  */
-	    || TREE_CODE (@2) == INTEGER_CST))
+	    || poly_int_tree_p (@4)))
      (if (TYPE_OVERFLOW_WRAPS (TREE_TYPE (@1)))
        (op @1 (convert @2))
        (with { tree utype = unsigned_type_for (TREE_TYPE (@1)); }

gcc/testsuite/gcc.target/aarch64/sve/acle/general/cntb_1.c

@@ -0,0 +1,14 @@
+/* { dg-options "-O -fdump-tree-optimized" } */
+
+#include <arm_sve.h>
+
+unsigned int
+foo (unsigned int x)
+{
+  unsigned long tmp = x;
+  tmp += svcntb ();
+  x = tmp;
+  return x - svcntb ();
+}
+
+/* { dg-final { scan-tree-dump-not { POLY_INT_CST } optimized } } */