match.pd: Fix up fneg/fadd simplification [PR109230]

match.pd: Fix up fneg/fadd simplification [PR109230]

The following testcase is miscompiled on aarch64-linux.  match.pd
has a simplification for addsub, where it negates one of the vectors
in twice as large floating point element vector (effectively negating every
other element) and then doing addition.
But a requirement for that is that the permutation picks the right elements,
in particular 0, nelts+1, 2, nelts+3, 4, nelts+5, ...
The pattern tests this with sel.series_p (0, 2, 0, 2) check, which as
documented verifies that the even elements of the permutation mask are
identity, but doesn't say anything about the others.
The following patch fixes it by also checking that the odd elements
start at nelts + 1 with the same step of 2.

2023-03-26  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/109230
	* match.pd (fneg/fadd simplify): Verify also odd permutation indexes.

	* gcc.dg/pr109230.c: New test.