[19/77] Add a smallest_int_mode_for_size helper function
This patch adds a wrapper around smallest_mode_for_size
for cases in which the mode class is MODE_INT. Unlike
(int_)mode_for_size, smallest_mode_for_size always returns
a mode of the specified class, asserting if no such mode exists.
smallest_int_mode_for_size therefore returns a scalar_int_mode
rather than an opt_scalar_int_mode.
2017-08-30 Richard Sandiford <richard.sandiford@linaro.org>
Alan Hayward <alan.hayward@arm.com>
David Sherwood <david.sherwood@arm.com>