InstCombine: move hasOneUse check to the top of foldICmpAddConstant

There were two combines not covered by the check before now, neither of which
actually differed from normal in the benefit analysis.

The most recent seems to be because it was just added at the top of the
function (naturally). The older is from way back in 2008 (r46687) when we just
didn't put those checks in so routinely, and has been diligently maintained
since.

llvm-svn: 341831