Reapply "[RISCVGatherScatterLowering] Minor code cleanup [NFC]"

This was reverted in 4e84149c due to a problem report which has been confirmed to be misattributed.

Original commit message:

Use a switch to avoid repeat tests on the opcode, and factor out some common code out of another switch.  (Not all branches had both common bits, but the one which didn't left the values unchanged and their starting value is the respective operand - so storing it back is a nop.)