[3/3] [AArch64][vect] vec_widen_lshift pattern

Add aarch64 vec_widen_lshift_lo/hi patterns and fix bug it triggers in
mid-end. This pattern takes one vector with N elements of size S, shifts
each element left by the element width and stores the results as N
elements of size 2*s (in 2 result vectors). The aarch64 backend
implements this with the shll,shll2 instruction pair.

gcc/ChangeLog:

	* config/aarch64/aarch64-simd.md: Add vec_widen_lshift_hi/lo<mode>
	patterns.
	* tree-vect-stmts.c (vectorizable_conversion): Fix for widen_lshift
	case.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/vect-widen-lshift.c: New test.