aarch64: Check CONSTM1_RTX in definition of Dm constraint

The constraint Dm is intended to match vectors of minus 1, but actually
checks for CONST1_RTX. This doesn't have a bad effect in practice as its
only use in the aarch64_wrffr pattern for the setffr instruction which
is a VNx16BI operation and -1 and 1 are the same there. That pattern
can only be currently generated through intrinsics anyway that create it
with a CONSTM1_RTX constant.

Fix the constraint definition so that it doesn't become a footgun if its
used in some other pattern.

Bootstrapped and tested on aarch64-none-linux-gnu.

Signed-off-by: Kyrylo Tkachov <ktkachov@nvidia.com>

gcc/ChangeLog:

	* config/aarch64/constraints.md (Dm): Match CONSTM1_RTX rather
	CONST1_RTX.