ARM: VIXL32: Fix crash in Exchange for stack slots.

In ParallelMoveResolverARMVIXL::Exchange(int mem1, int mem2)
scratch general purpose register was used without any spilling
(like in StoreToOffset) which led to lack of scratch register
for VLDR with big offset. Now it uses two scratch S-registers.

Test: ART_USE_VIXL_ARM_BACKEND=true m test-art-host
Test: ART_USE_VIXL_ARM_BACKEND=true m test-art-target
Change-Id: I0416a69e281d09a04dd1689efa5a8c1994c82638