[CSKY] Fix warning in CSKYMCCodeEmitter::getImmJMPIX.

Use llvm_unreachable instead of assert and replace if-else with switch-case.

Fix llvm#97456.

Author: tclin914

llvm/lib/Target/CSKY/MCTargetDesc/CSKYMCCodeEmitter.cpp

@@ -278,16 +278,18 @@ CSKYMCCodeEmitter::getRegisterSeqOpValue(const MCInst &MI, unsigned Op,
 unsigned CSKYMCCodeEmitter::getImmJMPIX(const MCInst &MI, unsigned Idx,
                                         SmallVectorImpl<MCFixup> &Fixups,
                                         const MCSubtargetInfo &STI) const {
-  if (MI.getOperand(Idx).getImm() == 16)
+  switch (MI.getOperand(Idx).getImm()) {
+  default:
+    llvm_unreachable("Unhandled jmpix imm!");
+  case 16:
     return 0;
-  else if (MI.getOperand(Idx).getImm() == 24)
+  case 24:
     return 1;
-  else if (MI.getOperand(Idx).getImm() == 32)
+  case 32:
     return 2;
-  else if (MI.getOperand(Idx).getImm() == 40)
+  case 40:
     return 3;
-  else
-    assert(0);
+  }
 }
 
 MCFixupKind CSKYMCCodeEmitter::getTargetFixup(const MCExpr *Expr) const {