include/exec/memop: Rename get_alignment_bits

include/exec/memop: Rename get_alignment_bits

Rename to use "memop_" prefix, like other functions
that operate on MemOp.

Reviewed-by: Helge Deller <deller@gmx.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>