Trasitively requiring RegisterCoalescer in RegAllocLinearScan

Hi all,

A question for other regalloc hackers - is there any good reason why
we're requiring RegisterCoalescer tranisitively in RegAllocLinearScan
(via addRequiredTransitive) ?

If there's no reason I'll switch it to a plain requirement
(addRequired). It's not a big deal, but is a little cleaner.

Cheers,
Lang.