[OpenCL] Add separate read_only and write_only pipe IR types

SPIR-V encodes the read_only and write_only access qualifiers of pipes,
so separate LLVM IR types are required to target SPIR-V.  Other backends
may also find this useful.

These new types are `opencl.pipe_ro_t` and `opencl.pipe_wo_t`, which
replace `opencl.pipe_t`.

This replaces __get_pipe_num_packets(...) and __get_pipe_max_packets(...)
which took a read_only pipe with separate versions for read_only and
write_only pipes, namely:

 * __get_pipe_num_packets_ro(...)
 * __get_pipe_num_packets_wo(...)
 * __get_pipe_max_packets_ro(...)
 * __get_pipe_max_packets_wo(...)

These separate versions exist to avoid needing a bitcast to one of the
two qualified pipe types.

Patch by Stuart Brady.

Differential Revision: https://reviews.llvm.org/D46015

llvm-svn: 331026