libstdc++: Use __builtin_shufflevector for simd split and concat

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>

libstdc++-v3/ChangeLog:

	PR libstdc++/114958
	* include/experimental/bits/simd.h (__as_vector): Return scalar
	simd as one-element vector. Return vector from single-vector
	fixed_size simd.
	(__vec_shuffle): New.
	(__extract_part): Adjust return type signature.
	(split): Use __extract_part for any split into non-fixed_size
	simds.
	(concat): If the return type stores a single vector, use
	__vec_shuffle (which calls __builtin_shufflevector) to produce
	the return value.
	* include/experimental/bits/simd_builtin.h
	(__shift_elements_right): Removed.
	(__extract_part): Return single elements directly. Use
	__vec_shuffle (which calls __builtin_shufflevector) to for all
	non-trivial cases.
	* include/experimental/bits/simd_fixed_size.h (__extract_part):
	Return single elements directly.
	* testsuite/experimental/simd/pr114958.cc: New test.