[llvm] [WIP][SPARC] Mark branches as being expensive in early Niagara CPUs (PR #166489)

[via llvm-commits]

koachan wrote:

> Setting these flags doesn't necessarily result in better codegen (in my experience). The impact should be assessed using some benchmarks.

Ya, that's why it's still marked as WIP.
But before doing benchmarks I at least want to confirm that the codegen changes are actually happening, hence the test case.

https://github.com/llvm/llvm-project/pull/166489