[X86] Remove X86RegisterInfo::getSEHRegNum. (#106866)

As far as I can tell, there's no way to call this. There are no calls in
the X86 directory. It has the same name as a function in MCRegisterInfo,
but that function takes a MCRegister and isn't virtual.

The function in MCRegisterInfo uses a DenseMap populated by
`X86_MC::initLLVMToSEHAndCVRegMapping`. The DenseMap is populated for
every physical register using the encoding value. I think that means the
function in MCRegisterInfo would return the same value as the function
in X86RegisterInfo.

Author: topperc

llvm/lib/Target/X86/X86RegisterInfo.cpp

@@ -83,11 +83,6 @@ X86RegisterInfo::X86RegisterInfo(const Triple &TT)
   }
 }
 
-int
-X86RegisterInfo::getSEHRegNum(unsigned i) const {
-  return getEncodingValue(i);
-}
-
 const TargetRegisterClass *
 X86RegisterInfo::getSubClassWithSubReg(const TargetRegisterClass *RC,
                                        unsigned Idx) const {

llvm/lib/Target/X86/X86RegisterInfo.h

@@ -54,9 +54,6 @@ class X86RegisterInfo final : public X86GenRegisterInfo {
   /// Return the number of registers for the function.
   unsigned getNumSupportedRegs(const MachineFunction &MF) const override;
 
-  // FIXME: This should be tablegen'd like getDwarfRegNum is
-  int getSEHRegNum(unsigned i) const;
-
   /// getMatchingSuperRegClass - Return a subclass of the specified register
   /// class A so that each register in it has a sub-register of the
   /// specified sub-register index which is in the specified register class B.