[AArch64] Robustify aarch64_wrffr

[AArch64] Robustify aarch64_wrffr

This patch uses distinct values for the FFR and FFRT outputs of
aarch64_wrffr, so that a following aarch64_copy_ffr_to_ffrt has
an effect.  This is needed to avoid regressions with later patches.

The block comment at the head of the file already described
the pattern this way, and there was already an unspec for it.
Not sure what made me change it...

2019-11-16  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64-sve.md (aarch64_wrffr): Wrap the FFRT
	output in UNSPEC_WRFFR.

From-SVN: r278356