[llvm] [RISCV] Consistently use RISCVSubtarget::is64Bit() instead of isRV32/isRV64. (PR #223038)

[Alexander Richardson via llvm-commits]

https://github.com/arichardson approved this pull request.

I agree that isRV32/64 is probably cleaner but IMO the most important is to be consistent

https://github.com/llvm/llvm-project/pull/223038