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.

1 files changed, 37 insertions, 0 deletions

diff --git a/test/zero_displacement.asm b/test/zero_displacement.asm
new file mode 100644
index 0000000..5aa4743
--- /dev/null
+++ b/test/zero_displacement.asm
@@ -0,0 +1,37 @@
+;Testname=O0; Arguments=-O0 -fbin -ozero_displacement.bin; Files=stdout stderr zero_displacement.bin
+;Testname=OL; Arguments=-OL -fbin -ozero_displacement.bin; Files=stdout stderr zero_displacement.bin
+;Testname=O1; Arguments=-O1 -fbin -ozero_displacement.bin; Files=stdout stderr zero_displacement.bin
+;Testname=Ox; Arguments=-Ox -fbin -ozero_displacement.bin; Files=stdout stderr zero_displacement.bin
+
+bits 16
+
+mov ax, [bx]
+mov ax, [bx+0]
+
+mov ax, [bx+di]
+mov ax, [bx+di+0]
+
+mov ax, [bp]
+mov ax, [bp+0]
+
+bits 32
+
+mov eax, [eax]
+mov eax, [eax+0]
+
+mov eax, [eax+ebx]
+mov eax, [eax+ebx+0]
+
+mov eax, [ebp]
+mov eax, [ebp+0]
+
+bits 64
+
+mov eax, [rax]
+mov eax, [rax+0]
+
+mov eax, [rax+rbx]
+mov eax, [rax+rbx+0]
+
+mov eax, [rbp]
+mov eax, [rbp+0]