Verify LLVM output in ptx unit tests

[vosen]

Expected outcome:
For each .ptx file that is unit-tested by the ptx project there is a textual LLVM output that is also verified (as a separate Rust unit tests)

Comments:

• Last time I checked LLVM does not ship ability to compare Module objects (and implementing it ourselves would be complex due to all the details). It's sufficient to just compare strings

[JoelleJS]

I'd like to have a go at this

[vosen]

Excelellent, assigned.
If the compiled LLVM module does not match the reference then it should be printed to output. If there's the a variable set (pick a name you like) that points to a directory then it also is written to the directory.
Typical use case is that someone makes a minor change that affects a large number of tests (perhaps adding an attribute) and would like to diff all of the failed tests against reference