[InstCombine] Use CaptureTracking in foldAllocaCmp()

foldAllocaCmp() checks whether the alloca is not captured (ignoring
the icmp). Replace the manual implementation of escape analysis
with CaptureTracking.

The primary practical difference is that CaptureTracking handles
nocapture arguments, while foldAllocaCmp() was using a hardcoded
list.

This is basically just the CaptureTracking refactoring from D120371
without the other changes.