[RISCV] Move FixableDef handling out of isSignExtendingOpW.

We have two layers of opcode checks. The first is in
isSignExtendingOpW. If that returns false, a second switch is used
for looking through nodes by adding them to the worklist.

Move the FixableDef handling to the second switch. This simplies
the interface of isSignExtendingOpW and makes that function more
accurate to its name.