Correct the shift used to select condition bits and correctly compute the
branch offset.

Sign extension shift had an off-by-one error so the sign bit was not being
properly extended.

T4 encodings of unconditional branches were not being interpreted correctly
(#6), and 32-bit bl/blx instructions were similarly incorrect. Correct the bits
selected for op1 and op2 and handle the slightly-unusual i1/i2 sign bit xor for
these instructions.