[DebugInfo] Make DIEDelta::SizeOf() more explicit. NFCI.

[ikudrin]

Summary

The patch restricts DIEDelta::SizeOf() to accept only DWARF forms that are actually used in the LLVM codebase. This should make the use of the class more explicit and help to avoid issues similar to fixed in D83958 and D84094.

[dblaikie]

Sounds good - thanks!

(I'm of slightly mixed feelings about this sort of overly narrowing of contracts if some code can reasonably be agnostic/general without a great deal of work - but they're easy enough to widen again if/when needed I suppose)

llvm/lib/CodeGen/AsmPrinter/DIE.cpp

 ///
 void DIEDelta::emitValue(const AsmPrinter *AP, dwarf::Form Form) const {
   AP->emitLabelDifference(LabelHi, LabelLo, SizeOf(AP, Form));
 }
 
 /// SizeOf - Determine size of delta value in bytes.
 ///
 unsigned DIEDelta::SizeOf(const AsmPrinter *AP, dwarf::Form Form) const {
-  if (Form == dwarf::DW_FORM_data4) return 4;
-  if (Form == dwarf::DW_FORM_sec_offset) return 4;
-  if (Form == dwarf::DW_FORM_strp) return 4;
-  return AP->MAI->getCodePointerSize();
+  switch (Form) {
+  case dwarf::DW_FORM_data4:
+    return 4;
+  case dwarf::DW_FORM_sec_offset:
+    // FIXME: add support for DWARF64
+    return 4;
+  default:
+    llvm_unreachable("DIE Value form not supported yet");
+  }
 }
 
 LLVM_DUMP_METHOD
 void DIEDelta::print(raw_ostream &O) const {
   O << "Del: " << LabelHi->getName() << "-" << LabelLo->getName();
 }
 
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