InstCombine: Process addrspacecast uses in PointerReplacer (#91953)

This was looking through an addrspacecast, and not finding a later
unfoldable cast to another address space. Fixes improperly deleting
a required alloca + memcpy and introducing an illegal addrspacecast.
    
This also required fixing some worklist management issues with
addrspacecast, and assuming that only memcpy sources could need
replacement.
    
Regresses one test function, but this looks like it optimized
before by accident. It never saw the pointer use by the call
to readonly_callee, which should require insertion of a new cast.
    
Fixes #68120