[RISCV] Remove RISCVISD opcodes for LGA, LA_TLS_IE, and LA_TLS_GD. (#70137)

This effectively reverts f912d21e

This was originally done for consistency with RISCVISD::ADD_LO so that
all nodes were emitted as RISCVISD nodes.

I've received feedback a couple times that its not worth it. So I'm
putting it back.