Part 1 of 3 patches that completes very long conditional branches

Part 1 of 3 patches that completes very long conditional branches
in constant islands for Mips16. We introdcuce JalB16 as a synomnym
for Jal16. It makes it easier to read and is also necessary because
Jal16 is a call instruction but JalB16 is being used as a branch.
Various parts of LLVM will not work properly even in this late stage of
the backend if we use what was declared as a call instruction to function
as a branch. For one, basic block labels may not get emitted in some
situations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195968 91177308-0d34-0410-b5e6-96231b3b80d8