[ARM][Mips][PowerPC] Remove unnecessary static_cast creating GISel InstructionSelector. NFC

Some targets only pass a TargetMachine & to their subtarget constructor
and require a static_cast to their target-specific TargetMachine subclass
to create *InstructionSelector.

These 3 targets already have the correct TargetMachine subclass
reference so no cast is needed.

Author: topperc

llvm/lib/Target/ARM/ARMSubtarget.cpp

@@ -111,8 +111,7 @@ ARMSubtarget::ARMSubtarget(const Triple &TT, const std::string &CPU,
   // FIXME: At this point, we can't rely on Subtarget having RBI.
   // It's awkward to mix passing RBI and the Subtarget; should we pass
   // TII/TRI as well?
-  InstSelector.reset(createARMInstructionSelector(
-      *static_cast<const ARMBaseTargetMachine *>(&TM), *this, *RBI));
+  InstSelector.reset(createARMInstructionSelector(TM, *this, *RBI));
 
   RegBankInfo.reset(RBI);
 }

llvm/lib/Target/Mips/MipsSubtarget.cpp

@@ -217,8 +217,7 @@ MipsSubtarget::MipsSubtarget(const Triple &TT, StringRef CPU, StringRef FS,
 
   auto *RBI = new MipsRegisterBankInfo(*getRegisterInfo());
   RegBankInfo.reset(RBI);
-  InstSelector.reset(createMipsInstructionSelector(
-      *static_cast<const MipsTargetMachine *>(&TM), *this, *RBI));
+  InstSelector.reset(createMipsInstructionSelector(TM, *this, *RBI));
 }
 
 bool MipsSubtarget::isPositionIndependent() const {

llvm/lib/Target/PowerPC/PPCSubtarget.cpp

@@ -65,8 +65,7 @@ PPCSubtarget::PPCSubtarget(const Triple &TT, const std::string &CPU,
   auto *RBI = new PPCRegisterBankInfo(*getRegisterInfo());
   RegBankInfo.reset(RBI);
 
-  InstSelector.reset(createPPCInstructionSelector(
-      *static_cast<const PPCTargetMachine *>(&TM), *this, *RBI));
+  InstSelector.reset(createPPCInstructionSelector(TM, *this, *RBI));
 }
 
 void PPCSubtarget::initializeEnvironment() {