[PowerPC] Let base implementation decide if MI is rematerizable by default (llvm#75772)

If MI is not PPC specific instructions, let base implementation decide
if MI is rematerizable.
This can fix failure in llvm#75570 after llvm#75271 .

Author: Kai Luo

llvm/lib/Target/PowerPC/PPCInstrInfo.cpp

@@ -1067,9 +1067,7 @@ bool PPCInstrInfo::isReallyTriviallyReMaterializable(
     const MachineInstr &MI) const {
   switch (MI.getOpcode()) {
   default:
-    // This function should only be called for opcodes with the ReMaterializable
-    // flag set.
-    llvm_unreachable("Unknown rematerializable operation!");
+    // Let base implementaion decide.
     break;
   case PPC::LI:
   case PPC::LI8: