MCAsmBackend: Use assert for unreachable relaxInstruction dump code

The check duplicates what fixupNeedsRelaxationAdvanced has checked.
The dump code is not very useful as it does not show the instruction
name.

Author: MaskRay

llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp

@@ -329,15 +329,7 @@ bool ARMAsmBackend::fixupNeedsRelaxationAdvanced(const MCFixup &Fixup,
 void ARMAsmBackend::relaxInstruction(MCInst &Inst,
                                      const MCSubtargetInfo &STI) const {
   unsigned RelaxedOp = getRelaxedOpcode(Inst.getOpcode(), STI);
-
-  // Return a diagnostic if we get here w/ a bogus instruction.
-  if (RelaxedOp == Inst.getOpcode()) {
-    SmallString<256> Tmp;
-    raw_svector_ostream OS(Tmp);
-    Inst.dump_pretty(OS);
-    OS << "\n";
-    report_fatal_error("unexpected instruction to relax: " + OS.str());
-  }
+  assert(RelaxedOp != Inst.getOpcode());
 
   // If we are changing Thumb CBZ or CBNZ instruction to a NOP, aka tHINT, we
   // have to change the operands too.

llvm/lib/Target/M68k/MCTargetDesc/M68kAsmBackend.cpp

@@ -222,15 +222,7 @@ bool M68kAsmBackend::fixupNeedsRelaxation(const MCFixup &Fixup,
 void M68kAsmBackend::relaxInstruction(MCInst &Inst,
                                       const MCSubtargetInfo &STI) const {
   unsigned RelaxedOp = getRelaxedOpcode(Inst.getOpcode());
-
-  if (RelaxedOp == Inst.getOpcode()) {
-    SmallString<256> Tmp;
-    raw_svector_ostream OS(Tmp);
-    Inst.dump_pretty(OS);
-    OS << "\n";
-    report_fatal_error("unexpected instruction to relax: " + OS.str());
-  }
-
+  assert(RelaxedOp != Inst.getOpcode());
   Inst.setOpcode(RelaxedOp);
 }
 

llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp

@@ -754,15 +754,7 @@ void X86AsmBackend::relaxInstruction(MCInst &Inst,
   // The only relaxations X86 does is from a 1byte pcrel to a 4byte pcrel.
   bool Is16BitMode = STI.hasFeature(X86::Is16Bit);
   unsigned RelaxedOp = getRelaxedOpcode(Inst, Is16BitMode);
-
-  if (RelaxedOp == Inst.getOpcode()) {
-    SmallString<256> Tmp;
-    raw_svector_ostream OS(Tmp);
-    Inst.dump_pretty(OS);
-    OS << "\n";
-    report_fatal_error("unexpected instruction to relax: " + OS.str());
-  }
-
+  assert(RelaxedOp != Inst.getOpcode());
   Inst.setOpcode(RelaxedOp);
 }