aarch64: Handle more SVE vector constants [PR99246]

aarch64: Handle more SVE vector constants [PR99246]

PR99246 is about a case in which we failed to handle a CONST_VECTOR
with NELTS_PER_PATTERN==2, i.e. a vector with a “foreground” sequence
of N vectors followed by a repeating “background” sequence of N vectors.

At the moment, it's difficult to produce these vectors directly,
but I'm hoping that for GCC 12 we'll do more folding, which will
in turn make this easier to test and easier to optimise.  Until then,
the patch simply relies on the testcase in the PR.

gcc/
	PR target/99246
	* config/aarch64/aarch64.c (aarch64_expand_sve_const_vector_sel):
	New function.
	(aarch64_expand_sve_const_vector): Use it for nelts_per_pattern==2.

gcc/testsuite/
	PR target/99246
	* gcc.target/aarch64/sve/acle/general/pr99246.c: New test.