[mlir][llvm] Make LoopAnnotations non-discardable

This commit adds the loop annotation attribute to LLVM::Br and
LLVM::CondBr to ensure it is non-discardable. Furthermore, the name is
changed from "llvm.loop" to "loop-annotation".

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D143986