[RISCV] Remove FCSR from RISCVRegisterInfo.

We only used this to mark it as a reserved register. But that's not
important if we don't do anything else with it.

I think if we were ever to do anything with it, we would need to
model it as a super register of FRM and FFLAGS. But it might be
easier to reference both FRM and FFLAGS in implicit defs/uses
for anything we were to do with "fcsr".

Reviewed By: sepavloff

Differential Revision: https://reviews.llvm.org/D115455

llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp

@@ -105,7 +105,6 @@ BitVector RISCVRegisterInfo::getReservedRegs(const MachineFunction &MF) const {
   // Floating point environment registers.
   markSuperRegs(Reserved, RISCV::FRM);
   markSuperRegs(Reserved, RISCV::FFLAGS);
-  markSuperRegs(Reserved, RISCV::FCSR);
 
   assert(checkAllSuperRegsMarked(Reserved));
   return Reserved;

llvm/lib/Target/RISCV/RISCVRegisterInfo.td

@@ -577,7 +577,6 @@ foreach m = LMULList.m in {
 // Special registers
 def FFLAGS : RISCVReg<0, "fflags">;
 def FRM    : RISCVReg<0, "frm">;
-def FCSR   : RISCVReg<0, "fcsr">;
 
 // Any type register. Used for .insn directives when we don't know what the
 // register types could be.