Change the default branch instruction to be the 16 bit variety for mips16.

Change the default branch instruction to be the 16 bit variety for mips16.
This has no material effect at this time since we don't have a direct
object emitter for mips16 and the assembler can't tell them apart. I
place a comment "16 bit inst" for those so that I can tell them apart in the
output. The constant island pass has only been minimally changed to allow
this. More complete branch work is forthcoming but this is the first
step.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194442 91177308-0d34-0410-b5e6-96231b3b80d8