PATCH. Fix ARM Leapcrel definition to make it loop hoistable

[Tim Northover]

Hi Yin,

> This patch enables LEAPCREL to be moved out of a loop because it is
> currently marked as hasSideEffects in .td file. Actually, it is not. By reading
> the comment above this instruction. It should not be marked hasSideEffects
> from the very beginning.

Could you explain why Jakob's comment in r162603's commit message
(when he added the "hasSideEffects = 1" line) is no longer valid?

Also, as usual, tests would be good.

Cheers.

Tim.