[llvm] [RISCV] Merge machine operand flag MO_PLT into MO_CALL (PR #77253)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Thanks. This is deliberated separated from previous patches like #72467 to keep earlier changes small. This has been on the road map for quite some time. I'll merge this now...

https://github.com/llvm/llvm-project/pull/77253