Switch on the use of arbitrary precision integers in scalar evolution. This will

Switch on the use of arbitrary precision integers in scalar evolution. This will
bail after 256-bits to avoid producing code that the backends can't handle.
Previously, we capped it at 64-bits, preferring to miscompile in those cases.

This change also reverts much of r52248 because the invariants the code was
expecting are now being met.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@53812 91177308-0d34-0410-b5e6-96231b3b80d8