Add tests for ByteAddressBuffer's templated load/store member functions

[bogner]

While we test the basic functionality of ByteAddressBuffer in https://github.com/llvm/offload-test-suite/blob/main/test/Feature/RawBuffers/ByteAddressBuffers.test, we need to also test templated loads and stores. These member functions are described here: https://github.com/microsoft/DirectXShaderCompiler/wiki/ByteAddressBuffer-Load-Store-Additions

[pow2clk]

The tests I added to DXC recently might serve as a template for these. They are those with the name buffer*-load-store* in this directory

Of particular interest might be the aggregates test, scalars test, and vectors test. Of these, I think the nuances of the aggregates test is the most likely to be missed as they involve multiple loads in many cases.

Missing is testing of 6.2 and previous style load/stores, which use a different opcode entirely as these tests were written to test later behavior.

[raoanag]

Working on this