i#1551 port to ARM: implement branch query routines

Implements the branch query routines instr_is_{call,cbr,mbr,ubr} for ARM.
Eliminates the opcode_is_() versions (as we need to examine operands or
predicates on ARM), replacing them with _arch() versions
that can assume the opcode is decoded.

Adds new Thumb opcodes OP_cbz and OP_b_short.  OP_b_short splits the 16-bit
encoding ("b.n") from the 32-bit ("b.w"), as automatically figuring out
which to use at encode time would essentially implement OP_jmp_short which
is non-trivial (i#56).

Implements instr_is_cit_short().  We'll need to mangle OP_cbz and OP_cbnz
like we do for OP_jecxz today.

Implements instr_is_return(), which we define as either read lr and write
pc or pop into pc.

Review-URL: https://codereview.appspot.com/185540043