[aarch64][vect] Support V8QI->V8HI WIDEN_ patterns

[aarch64][vect] Support V8QI->V8HI WIDEN_ patterns

In the case where 8 out of every 16 elements are widened using a
widening pattern and the next 8 are skipped, the patterns are not
recognized. This is because they are normally used in a pair, such  as
VEC_WIDEN_MINUS_HI/LO, to achieve a v16qi->v16hi conversion for example.
This patch adds support for V8QI->V8HI patterns.

gcc/ChangeLog:

	PR tree-optimization/98772
	* optabs-tree.c (supportable_half_widening_operation): New function
	to check for supportable V8QI->V8HI widening patterns.
	* optabs-tree.h (supportable_half_widening_operation): New function.
	* tree-vect-stmts.c (vect_create_half_widening_stmts): New function
	to create promotion stmts for V8QI->V8HI widening patterns.
	(vectorizable_conversion): Add case for V8QI->V8HI.

gcc/testsuite/ChangeLog:

	PR tree-optimization/98772
	* gcc.target/aarch64/pr98772.c: New test.