[Bug target/82621] [6/7/8 Regression] wrong code with -Og -fgcse -fweb

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82621

--- Comment #3 from Segher Boessenkool <segher at gcc dot gnu.org> ---
Trying 77 -> 90:
   77: {r196:SI=0-r188:SI;ca:SI=leu(r188:SI,0);}
      REG_UNUSED ca:SI
   90: {r197:SI=r196:SI+r196:SI+ca:SI;clobber ca:SI;}
      REG_DEAD r196:SI
      REG_DEAD ca:SI
      REG_UNUSED ca:SI

So combine did everything correctly here (the unused was here at the
start already).  Where did it come from?