66 prefix is ignored if REX prefix is used

[nepx]

If the 66 prefix is followed by a REX prefix (even one sans REX.W), its effect appears to be negated.

0000000000000000 664101C0                        ADD R8D,EAX
0000000000000004 664103DC                        ADD EBX,R12D
0000000000000008 664189C0                        MOV R8D,EAX
000000000000000C 66418BC0                        MOV EAX,R8D
0000000000000010 6641C1C001                      ROL R8D,01
0000000000000015 664189C0                        MOV R8D,EAX

Expected output (objdump):

0:  66 41 01 c0             add    r8w,ax
4:  66 41 03 dc             add    bx,r12w
8:  66 41 89 c0             mov    r8w,ax
c:  66 41 8b c0             mov    ax,r8w
10: 66 41 c1 c0 01          rol    r8w,0x1
15: 66 41 89 c0             mov    r8w,ax 

Interestingly enough, if the REX prefix comes first, then everything works as expected:

0000000000000000 416601C0                        ADD R8W,AX
0000000000000004 416603DC                        ADD BX,R12W
0000000000000008 416689C0                        MOV R8W,AX
000000000000000C 41668BC0                        MOV AX,R8W
0000000000000010 4166C1C001                      ROL R8W,01
0000000000000015 416689C0                        MOV R8W,AX

[Recoskie]

When the width bit was not set in the REX prefix, it defaulted to 32-bit. The update defaults to the current bit mode setting if the width bit is not set. Excellent work on spotting the issue, and thank you for reporting it.