Revert "[IndVars] Remove monotonic checks with unknown exit count"

This reverts commit c6ca26c0.
This breaks stage2 builds due to hitting this assert:
```
   Assertion failed: (WeightSum <= UINT32_MAX && "Expected weights to scale down to 32 bits"), function calcMetadataWeights
```
when compiling AArch64RegisterBankInfo.cpp in LLVM.