[llvm] [GlobalISel][Mips] Global ISel for `brcond` (PR #81306)

[via llvm-commits]

darkbuck wrote:

> LGTM. Maybe should change the description to say this is moving it to tablegen instead of suggesting it's new support

Revise the commit message following the suggestion. But, it seems `spr land` uses the one posted here and discards the local change. Learned that `spr diff` is required to if any change (including commit message) before `spr land`.

https://github.com/llvm/llvm-project/pull/81306