[LoopUnrollAnalyzer] Fix a crash in analyzeLoopUnrollCost.

In some cases, when simplifying with SCEV, we might consider pointer values as
just usual integer values.  Thus, we might get a different type from what we
had originally in the map of simplified values, and hence we need to check
types before operating on the values.

This fixes PR28015.

llvm-svn: 271931