x86-64: avoid bogus warnings with 32-bit addressing

x86-64: avoid bogus warnings with 32-bit addressing

With optimize_disp() adjusting i.types[].bitfield.disp after adjusting
the value to be used as displacement, it better also stores the updated
value, to avoid subsequent "... shortened to ..." warnings. Note how
optimize_imm() already does so.

The -0xffffffff tests being added expose a separate issue: The encoding
chosen should be 1 for ModR/M.mod, not 2. This will want to be taken
care of, but not right here.

This at the same time addresses a similar warning and demonstrates a
similar encoding issue with 16-bit addressing. Since it was omitted
when introducing the lea16-optimize test, add a plain lea16 one to also
cover this.