ARM: Teach assembler to enforce constraints for ARM LDRD destination register operands.

As specified in A8.8.72/A8.8.73/A8.8.74 in the ARM ARM, all variants of the ARM LDRD instruction have the following two constraints:

LDRD<c> <Rt>, <Rt2>, ...

(a) Rt must be even-numbered and not r14
(b) Rt2 must be R(t+1)

If those two constraints are not met the result of executing the instruction will be unpredictable.

Constraint (b) was already enforced, this commit adds support for constraint (a).

Fixes rdar://14479793.