target-mips: add mips16 instruction decoding

target-mips: add mips16 instruction decoding

There's no good way to add this incrementally, so we do it all at once.
The only changes to shared code are in handle_delay_slot.  We need to
flip ISAMode when doing a jump-and-exchange.  We also need to set
ISAMode the low bit of the target address for jump-to-register.

Also, since we're now adding bits that can be in MIPS_HFLAG_BMASK_EXT,
make sure we use MIPS_HFLAG_BMASK_BASE in the places where we just want
basic information about a branch.

Signed-off-by: Nathan Froyd <froydnj@codesourcery.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>