[LLParser] Remove special handling for call address space

Spin-off from D104740: I don't think this special handling is needed
anymore. Calls in textual IR are annotated with addrspace(N) (which
defaults to the program address space from data layout) and specifies
the expected pointer address space of the callee. There is no need
to special-case the program address space on top of that, as it
already is the default expected address space, and we shouldn't
allow use of the program address space if the call was explicitly
annotated with some other address space.

The IsCall parameter is retained because it will be used again soon.

Differential Revision: https://reviews.llvm.org/D104752