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