[RISCV] Check subtarget feature in getBrCond (#129859)
The function currently only checks to see if we compare against an
immediate before selecting the two branch immediate instructions that
are a part of the XCVbi vendor extension. This works at the moment since
there are no other extensions that have a branch immediate instruction.
It would be better if we explicitly check if the XCVbi extension is enabled
before returning the appropriate instruction.
This is also done in preparation for the branch immediate instructions
that are a part of the Xqcibi vendor extension from Qualcomm.
