[InstCombine] Replace two calls to getNumUses() with !hasNUsesOrMore

We were calling getNumUses to check for 1 or 2 uses. But getNumUses is linear in the number of uses. We can instead use !hasNUsesOrMore(3) which will stop the linear scan as soon as it determines there are at least 3 uses even if there are more.

llvm-svn: 340939