Improve process_ea and introduce -OL
Two fixes:
1. Optimization of [bx+0xFFFF] etc
0xFFFF is an sbyte under 16-bit semantics,
so make sure to check it right.
2. Don't optimize displacements in -O0
Displacements that fit into an sbyte or
can be removed should *not* be optimized in -O0.
Implicit zero displacements are still optimized, e.g.:
[eax] -> 0 bit displacement, [ebp] -> 8 bit displacement.
However explicit displacements are not optimized:
[eax+0] -> 32 bit displacement, [ebp+0] -> 32 bit displacement.
Because #2 breaks compatibility with 0.98,
I introduced a new optimization level: -OL, legacy.