[llvm] [LLVM][NVPTX] Add support for ldmatrix extensions introduced in PTX 8.6 (PR #124899)

[Artem Belevich via llvm-commits]

https://github.com/Artem-B approved this pull request.

LGTM.

Sorry about the delay. I apparently archived PR notification out of inbox before reading it.
In case you see no response from me in reasonable time in the future, feel free to ping me to bring it to my attention. Github should have some sort of opt-in auto-nag feature for assigned reviewers.

https://github.com/llvm/llvm-project/pull/124899