Fix GetMinTrailingZeros for SCEVSignExtend and SCEVZeroExtendExpr to

return the correct value when the cast operand is all zeros. This ought
to be pretty rare, because it would mean that the regular SCEV folding
routines missed a case, though there are cases they might legitimately
miss. Also, it's unlikely anything currently using GetMinTrailingZeros
cares about this case.

llvm-svn: 71532