[AArch64][SVE] Handle register-register pred_movs

[AArch64][SVE] Handle register-register pred_movs

pred_mov<mode> is defined for predicated loads and stores, where
exactly one of the operands is a register.  However, the instruction
condition only checked for "one" rather than "exactly one", and
Prathamesh found a case in which combine could fold a predicated
pattern to an all-register pred_mov<mode>.  The constraints would
then force one of the registers to memory.

This patch splits all-register forms into a normal move as soon
as possible, but also adds an all-register alternative in case the
instruction doesn't get split before RA (or in case the RA can use
inheritance to avoid a reload).

The testcase for this will be added to aarch64/sve-acle-branch.

2018-01-25  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64-sve.md (*pred_mov<mode>)
	(pred_mov<mode>): Handle all-register forms using both a new
	alternative and a split.

From-SVN: r268263