[AArch64] FPLoadBalancing: move ownership of the chain to its current accumulator register

and forget about the previously used accumulator.

Coming up with a simple testcase is not easy, as this highly depends on
what the register allocator is doing: this issue showed up while working
with the PBQP allocator, which produced a different allocation scheme.
A testcase would need to come up with chain starting in D[0-7], then
moving to D[8-15], followed by a call to a function whose regmask
clobbers the starting accumulator in D[0-7], then another use of the chain.

Fixed some formatting, added some invariant checks while there.