[InferAddressSpaces] Handle unconverted ptrmask (#140802)
In case a ptrmask cannot be converted to the new address space due to an
unknown mask value, this needs to be detcted and an addrspacecast is
needed to not hinder a future use of the unconverted return value of
ptrmask. Otherwise, users of this value will become invalid by receiving
a nullptr as an operand.
This LLVM defect was identified via the AMD Fuzzing project.
(See https://reviews.llvm.org/D80129 for an explanation of why some
ptrmasks are impossible to convert to other addrspaces.)
