[Unrolling] Refactor the start and step offsets to simplify overflow

[Unrolling] Refactor the start and step offsets to simplify overflow
checking and make the cache faster and smaller.

I had thought that using an APInt here would be useful, but I think
I was just wrong. Notably, we don't have to do any fancy overflow
checking, we can just bound the values as quite small and do the math in
a higher precision integer. I've switched to a signed integer so that
UBSan will even point out if we ever have integer overflow. I've added
various asserts to try to catch things as well and hoisted the overflow
checks so that we just leave the too-large offsets out of the SCEV-GEP
cache. This makes the value in the cache quite a bit smaller which is
probably worthwhile.

No functionality changed here (for trip counts under 1 billion).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237209 91177308-0d34-0410-b5e6-96231b3b80d8