loopz/loopnz clearing previous instruction's modified flags on cx -> 0

If you run QBasic in qemu, printing a double-type single-digit number will print an extra decimal point (e.g. PRINT CDBL(3) prints "3.") that does not appear when running on a real CPU (or on qemu with -enable-kvm). I tracked this down to the state of the status flags after a loopnz instruction.

After executing a sequence like this in qemu:

	mov bx,1
	mov cx,1
	dec bx    ; sets Z bit in flags
A:	loopnz A  ; should not modify flags

Z is incorrectly clear afterwards. loopz does the same thing (but not plain loop). Interestingly, inserting pushf+popf after dec results in Z set, so loopnz/loopz does not always clear Z itself but is rather interfering with the previous instruction's flag setting.

Version 5.1.0-rc0, x86-64 host.