Cache PrimExpr instead of raw pointers in bound analyzer (#5533)

Cache PrimExpr instead of raw pointers in bound analyzer (#5533)

The objects that the raw pointers point to can be deallocated and new
objects can be allocated at the same address, all while these pointers
are still in the cache. This can lead to unexpected behavior, for
example to calculated bound conflicts with previously cached values.

Caching PrimExpr will prevent the objects from being deallocated while
the cache is active.