Change the PhysRegsUsed map into a dense array. Seeing that this is a mapping

from physical registers, and they are always dense, it makes sense to not have
a ton of RBtree overhead.  This change speeds up regalloclocal about ~30% on
253.perlbmk, from .35s -> .27s in the JIT (in LLC, it goes from .74 -> .55).

Now live variable analysis is the slowest codegen pass.  Of course it doesn't
help that we have to run it twice, because regalloclocal doesn't update it,
but even if it did it would be the slowest pass (now it's just the 2x slowest
pass :(

llvm-svn: 11215