[RISCV] Simplify the check for when to call EmitLoweredCascadedSelect. NFC (#145930)

Based on the comments and tests, we only want to call
EmitLoweredCascadedSelect on selects of FP registers.

Everytime we add a new branch with immediate opcode, we've been
excluding it here.

This patch switches to checking that the comparison operands are both
registers so branch on immediate is automatically excluded.

Author: topperc

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -21445,12 +21445,8 @@ static MachineBasicBlock *emitSelectPseudo(MachineInstr &MI,
   // EmitLoweredCascadedSelect.
 
   auto Next = next_nodbg(MI.getIterator(), BB->instr_end());
-  if ((MI.getOpcode() != RISCV::Select_GPR_Using_CC_GPR &&
-       MI.getOpcode() != RISCV::Select_GPR_Using_CC_SImm5_CV &&
-       MI.getOpcode() != RISCV::Select_GPRNoX0_Using_CC_SImm5NonZero_QC &&
-       MI.getOpcode() != RISCV::Select_GPRNoX0_Using_CC_UImm5NonZero_QC &&
-       MI.getOpcode() != RISCV::Select_GPRNoX0_Using_CC_SImm16NonZero_QC &&
-       MI.getOpcode() != RISCV::Select_GPRNoX0_Using_CC_UImm16NonZero_QC) &&
+  if (MI.getOpcode() != RISCV::Select_GPR_Using_CC_GPR &&
+      MI.getOperand(1).isReg() && MI.getOperand(2).isReg() &&
       Next != BB->end() && Next->getOpcode() == MI.getOpcode() &&
       Next->getOperand(5).getReg() == MI.getOperand(0).getReg() &&
       Next->getOperand(5).isKill())