[DWARF] Simplify DWARFExpression. NFC.

[ikudrin]

Summary

As DataExtractor already has a method to extract an unsigned value of a specified size, there is no need to duplicate that.

[probinson]

LGTM, thanks for the simplification!
And it looks like SizeRefAddr could be simplified the same way, either as part of this patch or in a follow-up.

llvm/lib/DebugInfo/DWARF/DWARFExpression.cpp

case Operation::Size4:
       Operands[Operand] = Data.getU32(&Offset);
       if (Signed)
         Operands[Operand] = (int32_t)Operands[Operand];
       break;
     case Operation::Size8:
       Operands[Operand] = Data.getU64(&Offset);
       break;
     case Operation::SizeAddr:
-      if (AddressSize == 8) {
-        Operands[Operand] = Data.getU64(&Offset);
-      } else if (AddressSize == 4) {
-        Operands[Operand] = Data.getU32(&Offset);
-      } else {
-        assert(AddressSize == 2);
-        Operands[Operand] = Data.getU16(&Offset);
-      }
+      Operands[Operand] = Data.getUnsigned(&Offset, AddressSize);
       break;
     case Operation::SizeRefAddr:
       if (getRefAddrSize(AddressSize, Version) == 8) {
         Operands[Operand] = Data.getU64(&Offset);
       } else if (getRefAddrSize(AddressSize, Version) == 4) {
         Operands[Operand] = Data.getU32(&Offset);
       } else {
         assert(getRefAddrSize(AddressSize, Version) == 2);