[Matrix] Add single subscript test

[farzonl]

fixes #502
https://godbolt.org/z/xaEE44cbE
Test 3 cases (loads and stores):

1. single subscript as an l-value swizzle
2. single subscript where the store/load takes a vector r-value swizzle
3. single subscript where we store/load the vector as is.

[alsepkow]

very minor nit, but a for loop for 4 assignments with two of them be 'special' made me look at this for a moment longer.
4 explicit assignment statements might be cleaner.

[farzonl]

its going to be much more than 4 explicit asignments because its 16 asignments to fill up Out[i*4 + j].

[alsepkow]

LGTM. Minor nit on the first for loop in both tests. Take it or leave it.

[Icohedron]

LGTM! Just a few formatting/spacing nits

[tex3d]

Minor comments/suggestions, nothing necessarily requiring changes regarding:

• dynamic/static subscript indexing and potential optimizations
• symmetrical swizzles (equivalent when used in source or dest)

[tex3d]

Just a comment, not a request for change:

These subscript accesses look dynamic, but these loops are likely to be unrolled, either before IR (in DXC, Clang, etc.) or after (in a driver compiler). That unrolling would make all accesses static, even eliminating the intermediate local variables to copy values directly from static indices of In to static indices of Out. That's a fine path to test, but this might not test other important dynamic indexing code paths, if desired.

I guess this gets at the fuzzy purpose issue with the offload-test-suite. This seems fine to test this scenario end-to-end, whatever path it may take through optimizations, and maybe that's all that's desired for this test.

[farzonl]

I was trying to use uint GI : SV_GroupIndex for the input to main because I thought someone might mention that. But I could not get the persistance I needed to properly fill the out buffer.

In anycase the matrix execution tests are like you said more about confirming a matrix frontend features behave the same as they would on DXC and thus a little bit different than some of the other behaviors being tested in the offload test suite like intrinsic tests. In my case its more about layout and access of data which should be preserved regardless of optimization passes.