ARM: call thumb_pass_branch_condition() only for actual branch opcodes

Calling it first with every opcodes and then testing if the opcode
was indeed a branch instruction is wasteful and rather strange.
If ever thumb_pass_branch_condition() has side effects (say, like
printing a debugging traces) then the result would be garbage for most
Thumb instructions which have no condition code.

While at it, let's make the nearby code more readable by reducing some of
the redundant brace noise and reworking the error handling construct.

Signed-off-by: Nicolas Pitre <nico@marvell.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>