Add an option to enable the SSA based peephole optimizer.

Eventually this pass will provide substantially better code in the interim between when we
have a crappy isel and nice isel.  Unfortunately doing so requires fixing the backend to
actually SUPPORT all of the fancy addressing modes that we now generate, and writing a DCE
pass for machine code.  Each of these is a fairly substantial job, so this will remain disabled
for the immediate future.  :(

llvm-svn: 10276