[RISCV] Remove uses of RISCVII::hasMergeOp from RISCVDAGToDAGISel.cpp

This property was intended to indicate when RISCVAsmPrinter should
drop the tied source operand when converting to MCInst. Using it
in RISCVDAGToDAGISel distorts what it intended for.

This should remove some changes from D151850.

Reviewed By: frasercrmck, asb

Differential Revision: https://reviews.llvm.org/D152039