x86/APX: optimize certain {nf}-form insns to LEA

..., as that leaves EFLAGS untouched anyway. That's a shorter encoding,
available as long as certain constraints on operand size and registers
are met; see code comments.

Note that this requires deferring to derive encoding_evex from {nf}
presence, as in optimize_encoding() we want to avoid touching the insns
when {evex} was also used.

Note further that this requires want_disp32() to now also consider the
opcode: We don't want to replace i.tm.mnem_off, for diagnostics to still
report the original mnemonic (or else things can get confusing). While
there, correct adjacent mis-indentation.