arm: Use LDMIA/STMIA for thumb1 DI/DF loads/stores

arm: Use LDMIA/STMIA for thumb1 DI/DF loads/stores

If the address register is dead after load/store operation it looks
beneficial to use LDMIA/STMIA instead of pair of LDR/STR instructions,
at least if optimizing for size.

gcc/ChangeLog:

	* config/arm/arm.cc (thumb_load_double_from_address): Emit ldmia
	when address reg rewritten by load.
	* config/arm/thumb1.md (peephole2 to rewrite DI/DF load): New.
	(peephole2 to rewrite DI/DF store): New.
	* config/arm/iterators.md (DIDF): New.

gcc/testsuite:

	* gcc.target/arm/thumb1-load-store-64bit.c: Add new test.

Signed-off-by: Siarhei Volkau <lis8215@gmail.com>