Check in conditional branches for constant islands. Still need to finish

Check in conditional branches for constant islands. Still need to finish
conditional branches for very large targets. That will be the next small
patch. Everything now should in principle work as good (functionality
wise) as without constant islands so we decided at Mips/Imagination to
make constant islands the default for Mips16 now so that it will get
excercised a lot and this port is still experimentatl though hopefully soon
we will change the status. Some more cleanup and code review is in order
but things are converging fast.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195902 91177308-0d34-0410-b5e6-96231b3b80d8