ARM: Teach assembler to enforce constraint for Thumb2 LDRD (literal/immediate) destination register operands.

LDRD<c> <Rt>, <Rt2>, <label>
LDRD<c> <Rt>, <Rt2>, [<Rn>{, #+/-<imm>}]
LDRD<c> <Rt>, <Rt2>, [<Rn>], #+/-<imm>
LDRD<c> <Rt>, <Rt2>, [<Rn>, #+/-<imm>]!

As specified in A8.8.72/A8.8.73 in the ARM ARM, the T1 encoding has a constraint which enforces that Rt != Rt2.

If this constraint is not met the result of executing the instruction will be unpredictable.

Fixes rdar://14479780.