MIPS: Don't try to decode microMIPS branch instructions where they cannot exist.

In mm_isBranchInstr() we can short circuit the entire function if
!cpu_has_mmips.

Signed-off-by: David Daney <david.daney@cavium.com>
Acked-by: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5326/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>