[AArch64] Tighten move constraints for symbolic operands

[AArch64] Tighten move constraints for symbolic operands

The movsi and movdi constraints allowed the source to be any
absolute symbolic expression ("S").  That's OK for operands that
have already been vetted by the aarch64_mov_operand predicate but
causes problems if the register allocator substitutes an equivalence
(the usual "the constraints can't accept more than the predicates"
restriction).

Although all other uses of "S" in the backend are redundant and could
in principle be removed, "S" itself is a publicly-documented constraint
and so we'd have to keep its definition.  This patch therefore adds a
new "Usa" constraint for legitimate absolute address operands.

2017-05-08  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/constraints.md (Usa): New constraint.
	* config/aarch64/aarch64.md (*movsi_aarch64, *movdi_aarch64): Use it.

From-SVN: r247746