tcg/arm: Simplify usage of encode_imm

tcg/arm: Simplify usage of encode_imm

We have already computed the rotated value of the imm8
portion of the complete imm12 encoding.  No sense leaving
the combination of rot + rotation to the caller.

Create an encode_imm12_nofail helper that performs an assert.

This removes the final use of the local "rotl" function,
which duplicated our generic "rol32" function.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>