Don't rely on the fact that shift values are never very large, and thus

this substraction will result in small negative numbers at worst which
become very large positive numbers on assignment and are thus caught by
the <=4 check on the next line. The >0 check clearly intended to catch
these as negative numbers.

Spotted by inspection, and impossible to trigger given the shift widths
that can be used.

llvm-svn: 147773