aarch64: Fix dupq_* testsuite failures

This patch fixes the dupq_* testsuite failures.  The tests were
introduced with r15-3669-ga92f54f580c3 (which was a nice improvement)
and Pengxuan originally had a follow-on patch to recognise INDEX
constants during vec_init.

I'd originally wanted to solve this a different way, using wildcards
when building a vector and letting vector_builder::finalize find the
best way of filling them in.  I no longer think that's the best
approach though.  Stepped constants are likely to be more expensive
than unstepped constants, so we should first try finding an unstepped
constant that is valid, even if it has a longer representation than
the stepped version.

This patch therefore uses a variant of Pengxuan's idea.

While there, I noticed that the (old) code for finding an unstepped
constant only tried varying one bit at a time.  So for index 0 in a
16-element constant, the code would try picking a constant from index 8,
4, 2, and then 1.  But since the goal is to create "fewer, larger,
repeating parts", it would be better to iterate over a bit-reversed
increment, so that after trying an XOR with 0 and 8, we try adding 4
to each previous attempt, then 2 to each previous attempt, and so on.
In the previous example this would give 8, 4, 12, 2, 10, 6, 14, ...

The test shows an example of this for 8 shorts.

gcc/
	* config/aarch64/aarch64.cc (aarch64_choose_vector_init_constant):
	New function, split out from...
	(aarch64_expand_vector_init_fallback): ...here.  Use a bit-
	reversed increment to find a constant index.  Add support for
	stepped constants.

gcc/testsuite/
	* gcc.target/aarch64/sve/acle/general/dupq_12.c: New test.