aarch64: Use correct type attributes for RTL generating XTN(2)

Use the correct "neon_move_narrow_q" type attribute in RTL patterns
that generate XTN/XTN2 instructions.

This makes a material difference because these instructions can be
executed on both SIMD pipes in the Cortex-A57 core model, whereas the
"neon_shift_imm_narrow_q" attribute (in use until now) would suggest
to the scheduler that they could only execute on one of the two
pipes.

gcc/ChangeLog:

2021-05-18  Jonathan Wright  <jonathan.wright@arm.com>

	* config/aarch64/aarch64-simd.md: Use "neon_move_narrow_q"
	type attribute in patterns generating XTN(2).