testsuite: Fix vector-subscript-4.c [PR116421]

testsuite: Fix vector-subscript-4.c [PR116421]

Verify we don't have any vector temporaries in the IL at least until
ISEL which may introduce VEC_EXTRACTs on targets which support
non-constant indices (see PR116421).

As a pass I chose NRV for no particular reason except that it is
literally the last pass prior ISEL.  At least at time of writing this.

gcc/testsuite/ChangeLog:
	PR testsuite/116421
	* c-c++-common/vector-subscript-4.c: Check for vectors prior
	ISEL.