[llvm] [AMDGPU][MC] Restrict op_sel in VOP3P dot instructions (PR #100485)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

This matches what the documentation states. If we really want to support undocumented usage, we should make a more conscious effort to test and validate it really works 

https://github.com/llvm/llvm-project/pull/100485