[RISCV][NFC] Update RISCVInstrInfoC.td to match usual instruction naming convention

When an instruction mnemonic contains a '.', we usually name the instruction
with a _ in that place. e.g. fadd.s -> FADD_S.

This patch updates RISCVInstrInfoC.td to do the same, e.g. c.nop -> C_NOP.

Also includes some minor formatting changes in RISCVInstrInfoC.td to better
align it with the formatting conventions in the rest of the backend.

llvm-svn: 320560