Prior to this change spv_ptrcast (and OpBitcast) was never emitted for
GEP resulting pointers. While such SPIR-V was (mostly) accepted by the
NEO GPU driver, the generated SPIR-V was incorrect.
The newly added test (pointers/getelementptr-bitcast-load.ll) verifies
that a correct bitcast is added for more complex cases and passes
spirv-val. The test is based on an OpenCL CTS test (basic/prefetch).
This patch introduces a new spv_ptrcast intrinsic for tracking expected
pointer types. The change fixes multiple OpenCL CTS regressions due the
switch to opaque pointers (e.g. basic/hiloeo).
