[InstCombine] Teach alloca replacement to handle `addrspacecast`

- As the address space cast may not be valid on a specific target,
  `addrspacecast` is not handled when an `alloca` is able to be replaced
  with the source of memcpy/memmove. This patch addresses that by
  querying a target hook on whether that address space cast is valid.
  For example, on most GPU targets, the cast from a global pointer to a
  generic pointer is valid.
- If that cast is allowedd (by querying `isValidAddrSpaceCast`), the
  replacement is enhanced to handle that `addrspacecast` as well.

Reviewed By: yaxunl

Differential Revision: https://reviews.llvm.org/D147025