good in general, but bad nested loops

[Matthew Fluet]

Here's what's currently spy-ed in the tight loop for nestedloop.sml (the
older one, not the one Steve wrote today) under my current version of the
backend:

0x804aea0:      cmp    $0x0,%edx
0x804aea3:      je     0x804b0ec
0x804aea9:      dec    %edx
0x804aeaa:      jo     0x804b104
0x804aeb0:      inc    %ebx
0x804aeb1:      jo     0x804b114
0x804aeb7:      jmp    0x804aea0

And with -detect-overflow false:

0x804aaba:      cmp    $0x0,%ebp
0x804aabd:      je     0x804ad48
0x804aac3:      dec    %ebp
0x804aac4:      inc    %edx
0x804aac5:      jmp    0x804aaba

Can't ask for much better than that (except turning the cmp to a tstl).