[llvm] [RISCV, GISel] Unconditionally use MO_PLT for calls (PR #72355)

[Jessica Clarke via llvm-commits]

jrtc27 wrote:

I would be happy seeing SDAG changed too, and even having MO_CALL and MO_PLT merged into a single flag, with assembly always being parsed as using the PLT relocation. The pointless and somewhat broken relocation needs to die.

https://github.com/llvm/llvm-project/pull/72355