[OpenMP] Use IsHostPtr where needed in rest of omptarget.cpp

As started in D107925, this patch replaces the remaining occurrences
of `UNIFIED_SHARED_MEMORY && TgtPtrBegin == HstPtrBegin` in
`omptarget.cpp` with `IsHostPtr`.  The former condition is broken in
the rare case that the device and host happen to use the same address
for their mapped allocations.  I don't know how to write a test that's
likely to reveal this case.

Reviewed By: grokos

Differential Revision: https://reviews.llvm.org/D107928