[RISCV][GISel] Directly emit X0 from getICMPOperandsForBranch instead of using...

[RISCV][GISel] Directly emit X0 from getICMPOperandsForBranch instead of using buildConstant+selectConstant.

This is simpler to implement and matches what SelectionDAG emits.

Also allows us to make getICMPOperandsForBranch a static function.