[AArch64] Reorganise aarch64-sve.md

[AArch64] Reorganise aarch64-sve.md

aarch64-sve.md was getting a bit jumbled, with related operations
separated by unrelated operations.  Also, many SVE instructions need
to have several patterns due to the various ways in which predication
can be used, and it was getting hard to tell which instructions had a
complete set of patterns.

This patch therefore tries to use an explicit grouping scheme.
Banner comments are usually frowned on, but md files have been
a traditional exception (e.g. when using Mike's template for
new targets).

The patch also lists the instructions implemented by each section,
so that it's possible to search the file by instruction mnemonic.

I wouldn't be surprised if I end up having to rip out the contents
section, but I found it useful for the month or so that that I've
been using it locally.  The patch checks that the contents remain
up-to-date by running a checking script during an early stage of
the build.

No functional change intended.

2019-08-07  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64-sve.md: Reorganize contents and add
	banner comments.
	* config/aarch64/check-sve-md.awk: New file.
	* config/aarch64/t-aarch64 (s-check-sve-md): New rule.
	(insn-conditions.md): Depend on it.

From-SVN: r274184