match.pd: Relax rule to include POLY_INT_CSTs

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.