x86: assemble can accept invalid memory addressing modes.

[i509VCB]

Issuing the command !a x86 fld xword ptr [ax] (yes this is invalid) emits the following:

fld xword ptr [ax]  ; +0 = 67 db 28 

But then disassembling this sequence using !d x86 67 db 28 produces the following:

fld xword ptr [bx + si]  ; +0 = 67 db 28 

The assembler should be failing to assemble since [ax] is an invalid memory addressing mode in x86.

Similarly with an invalid mov ax, [ax] the following is generated:

mov ax, [ax]  ; +0 = 67 66 8b 00 

And disassembly shows:

mov ax, word ptr [bx + si]  ; +0 = 67 66 8b 00