Make first substantial checkin of my port of ARM constant islands code to Mips.

Make first substantial checkin of my port of ARM constant islands code to Mips.
Before I just ported the shell of the pass. I've tried to keep everything
nearly identical to the ARM version. I think it will be very easy to eventually
merge these two and create a new more general pass that other targets can
use. I have some improvements I would like to make to allow pools to
be shared across functions and some other things. When I'm all done we
can think about making a more general pass. More to be ported but the
basic mechanism works now almost as good as gcc mips16.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193509 91177308-0d34-0410-b5e6-96231b3b80d8