target-mips: mips16 cleanups

Change code handling mips16-specific branches to use ISA-neutral special
opcodes.  Since there are several places where the delay slot
requirements for microMIPS branches differ from mips16 branches, using
opcodes is easier than checking hflags, then checking mips16
vs. microMIPS.

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