[AArch64] Add banner comments to aarch64-sve2.md

[AArch64] Add banner comments to aarch64-sve2.md

This patch imposes the same sort of structure on aarch64-sve2.md
as we already have for aarch64-sve.md, before it grows a lot more
patterns.

2020-01-09  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64-sve2.md: Add banner comments.
	(<su>mulh<r>s<mode>3): Move further up file.
	(<su>mull<bt><Vwide>, <r>shrnb<mode>, <r>shrnt<mode>)
	(*aarch64_sve2_sra<mode>): Move further down file.
	* config/aarch64/t-aarch64 (s-check-sve-md): Check aarch64-sve2.md too.

From-SVN: r280058