[RISCV][NFC] Clean up RISCVDAGToDAGISel::Select

As pointed out in post-commit review of r318738, `return ReplaceNode(..)` when 
both ReplaceNode and the current function return void is confusing. This patch 
moves to using a more obvious early return, and moves to just using an if to 
catch the one case we currently care about. A future patch that adds further 
custom instruction selection can introduce a switch.

llvm-svn: 318757