ipa-sra: Testcase that removing a "returns_nonnull" retval works

Since we can now remove return values of functions with return_nonnull
type attribute, I'll feel a bit safer if we can test this does not ICE
when someone attempts to access a non-existent call LHS.  Eventually
we should probably drop the attribute when this happens.

gcc/testsuite/ChangeLog:

2021-11-15  Martin Jambor  <mjambor@suse.cz>

	* gcc.dg/ipa/ipa-sra-ret-nonull.c: New test.