[RISCV] Remove unused includes (NFC)

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

Author: kazutakahirata

llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCExpr.cpp

@@ -18,7 +18,6 @@
 #include "llvm/MC/MCAssembler.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCStreamer.h"
-#include "llvm/MC/MCSymbolELF.h"
 #include "llvm/MC/MCValue.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/ErrorHandling.h"

llvm/lib/Target/RISCV/RISCVSubtarget.cpp

@@ -17,8 +17,6 @@
 #include "RISCVFrameLowering.h"
 #include "RISCVSelectionDAGInfo.h"
 #include "RISCVTargetMachine.h"
-#include "llvm/CodeGen/MacroFusion.h"
-#include "llvm/CodeGen/ScheduleDAGMutation.h"
 #include "llvm/MC/TargetRegistry.h"
 #include "llvm/Support/ErrorHandling.h"
 

llvm/lib/Target/RISCV/RISCVVectorMaskDAGMutation.cpp

@@ -28,7 +28,6 @@
 
 #include "MCTargetDesc/RISCVBaseInfo.h"
 #include "MCTargetDesc/RISCVMCTargetDesc.h"
-#include "RISCVRegisterInfo.h"
 #include "RISCVTargetMachine.h"
 #include "llvm/CodeGen/LiveIntervals.h"
 #include "llvm/CodeGen/MachineInstr.h"