[APInt] Make toString use udivrem instead of calling the divide helper method directl...

[APInt] Make toString use udivrem instead of calling the divide helper method directly. Do a better job of reusing allocations while looping. NFCI

This lets toString take advantage of the degenerate case checks in udivrem and is just generally cleaner.

One minor downside of this is that the divisor APInt now needs to be the same size as Tmp which requires an additional allocation. But we were doing a poor job of reusing allocations before so the new code should still be an improvement.

llvm-svn: 302704