Tune coalescing in non-iterative register allocator

Tune coalescing in non-iterative register allocator

If iterative coalescing isn't turned on, then do a single aggressive
coalescing pass for the first build/color cycle and then back off to
conservative coalescing for subseqent passes.

Aggressive coalescing is a cheap way to eliminate lots of the reg-reg
moves, but it can make the graph less colorable - if we turn it on
for every pass then allocation for code with a large amount of register
pressure (ie SHA1) doesn't converge in a sensible number of cycles.