[RISCV][NFC] Rename callee saved regs 'CSR' to CSR_ILP32_LP64 and minor...

[RISCV][NFC] Rename callee saved regs 'CSR' to CSR_ILP32_LP64 and minor RISCVRegisterInfo refactoring

The CSR renaming further prepares the way for an upcoming patch adding support for more
RISC-V ABIs.

Modify RISCVRegisterInfo::getCalleeSavedRegs and
RISCVRegisterInfo::getReservedRegs to do MF->getSubtarget<RISCVSubtarget>()
once rather than multiple times.

llvm-svn: 356123