[AMDGPU] Remove unneed static_cast from GCNSubtarget constructor. NFC

RegBankInfo is a std::unique_ptr<AMDGPURegisterBankInfo> so we don't
need the cast.
diff --git a/llvm/lib/Target/AMDGPU/AMDGPUSubtarget.cpp b/llvm/lib/Target/AMDGPU/AMDGPUSubtarget.cpp
index b46adb9..21fe1bc 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUSubtarget.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUSubtarget.cpp
@@ -199,8 +199,7 @@
   InlineAsmLoweringInfo.reset(new InlineAsmLowering(getTargetLowering()));
   Legalizer.reset(new AMDGPULegalizerInfo(*this, TM));
   RegBankInfo.reset(new AMDGPURegisterBankInfo(*this));
-  InstSelector.reset(new AMDGPUInstructionSelector(
-  *this, *static_cast<AMDGPURegisterBankInfo *>(RegBankInfo.get()), TM));
+  InstSelector.reset(new AMDGPUInstructionSelector(*this, *RegBankInfo, TM));
 }
 
 unsigned GCNSubtarget::getConstantBusLimit(unsigned Opcode) const {