[AArch64] Add a aarch64_sve_mode_p query

[AArch64] Add a aarch64_sve_mode_p query

This patch adds an exported function for testing whether a mode is
an SVE mode.  The ACLE will make more use of it, but there's already
one place that can benefit.

2019-08-15  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64-protos.h (aarch64_sve_mode_p): Declare.
	* config/aarch64/aarch64.c (aarch64_sve_mode_p): New function.
	(aarch64_select_early_remat_modes): Use it.

From-SVN: r274523