arm: Fix LDRD register overlap [PR117675]

arm: Fix LDRD register overlap [PR117675]

The register indexed variants of LDRD have complex register overlap constraints
which makes them hard to use without using output_move_double (which can't be
used for atomics as it doesn't guarantee to emit atomic LDRD/STRD when required).
Add a new predicate and constraint for plain LDRD/STRD with base or base+imm.
This blocks register indexing and fixes PR117675.

gcc:
	PR target/117675
	* config/arm/arm.cc (arm_ldrd_legitimate_address): New function.
	* config/arm/arm-protos.h (arm_ldrd_legitimate_address): New prototype.
	* config/arm/constraints.md: Add new Uo constraint.
	* config/arm/predicates.md (arm_ldrd_memory_operand): Add new predicate.
	* config/arm/sync.md (arm_atomic_loaddi2_ldrd): Use
	arm_ldrd_memory_operand and Uo.

gcc/testsuite:
	PR target/117675
	* gcc.target/arm/pr117675.c: Add new test.