Fix the ldrd offsets of Thumb-2 - for TARGET_LDRD the range is +-1020,
without -252..4096.  This reduces the number of addressing instructions
when using DI mode operations (such as in PR77308).

    gcc/
	* config/arm/arm.c (arm_legitimate_index_p): Add comment.
	(thumb2_legitimate_index_p): Use correct range for DI/DF mode.

From-SVN: r251681