x86/APX: optimize {nf}-form IMUL-by-power-of-2 to SHL

..., for differing only in the resulting EFLAGS, which are left
untouched anyway. That's a shorter encoding, available as long as
certain constraints on operands are met; see code comments. (SHL-by-1
forms may then be subject to further optimization that was introduced
earlier.)

Note that kind of as a side effect this also converts multiplication by
1 to shift by 0, which is a plain move or even no-op anyway. That could
be further shrunk (as could be presence of shifts/rotates by 0 in the
original code as  well as a fair set of other {nf}-form insns), yet the
expectation (for now) is that people won't write such code in the first
place.