[Patch AArch64] GCC 6 regression in vector performance. - Fix vector initialization to happen with lane load instructions.

gcc/

	* config/aarch64/aarch64.c (aarch64_expand_vector_init): Refactor,
	always use lane loads to construct non-constant vectors.

gcc/testsuite/

	* gcc.target/aarch64/vector_initialization_nostack.c: New.


Co-Authored-By: Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>

From-SVN: r233461