[Bitcode][NFC] Add abbrev for FUNC_CODE_DEBUG_LOC

DILocations that are not attached to instructions are encoded using
METADATA_LOCATION records which have an abbrev. DILocations attached to
instructions are interleaved with instruction records as FUNC_CODE_DEBUG_LOC
records, which do not have an abbrev (and FUNC_CODE_DEBUG_LOC_AGAIN which have
no operands).

Add a new FUNCTION_BLOCK abbrev FUNCTION_DEBUG_LOC_ABBREV for
FUNC_CODE_DEBUG_LOC records.

This reduces the bc file size by up to 7% in CTMark, with many between 2-4%
smaller.

[per-file file size
compile-time-tracker](https://llvm-compile-time-tracker.com/compare.php?from=75cf826849713c00829cdf657e330e24c1a2fd03&to=1e268ebd0a581016660d9d7e942495c1be041f7d&stat=size-file&details=on)
(go to stage1-ReleaseLTO-g).

This optimisation is motivated by llvm#144102, which adds the new Key Instructions
fields to bitcode records. The combined patches still overall look to be a
slight improvement over the base.

Author: OCHyams

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -152,6 +152,7 @@ enum {
   FUNCTION_INST_UNREACHABLE_ABBREV,
   FUNCTION_INST_GEP_ABBREV,
   FUNCTION_DEBUG_RECORD_VALUE_ABBREV,
+  FUNCTION_DEBUG_LOC_ABBREV,
 };
 
 /// Abstract class to manage the bitcode writing, subclassed for each bitcode
@@ -3675,7 +3676,7 @@ void ModuleBitcodeWriter::writeFunction(
   // in the VST.
   FunctionToBitcodeIndex[&F] = Stream.GetCurrentBitNo();
 
-  Stream.EnterSubblock(bitc::FUNCTION_BLOCK_ID, 4);
+  Stream.EnterSubblock(bitc::FUNCTION_BLOCK_ID, 5);
   VE.incorporateFunction(F);
 
   SmallVector<unsigned, 64> Vals;
@@ -3724,7 +3725,8 @@ void ModuleBitcodeWriter::writeFunction(
           Vals.push_back(VE.getMetadataOrNullID(DL->getScope()));
           Vals.push_back(VE.getMetadataOrNullID(DL->getInlinedAt()));
           Vals.push_back(DL->isImplicitCode());
-          Stream.EmitRecord(bitc::FUNC_CODE_DEBUG_LOC, Vals);
+          Stream.EmitRecord(bitc::FUNC_CODE_DEBUG_LOC, Vals,
+                            FUNCTION_DEBUG_LOC_ABBREV);
           Vals.clear();
           LastDL = DL;
         }
@@ -4055,6 +4057,19 @@ void ModuleBitcodeWriter::writeBlockInfo() {
         FUNCTION_DEBUG_RECORD_VALUE_ABBREV)
       llvm_unreachable("Unexpected abbrev ordering! 1");
   }
+  {
+    auto Abbv = std::make_shared<BitCodeAbbrev>();
+    Abbv->Add(BitCodeAbbrevOp(bitc::FUNC_CODE_DEBUG_LOC));
+    // NOTE: No IsDistinct field for FUNCTION_DEBUG_LOCs.
+    Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6));
+    Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));
+    Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6));
+    Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6));
+    Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1));
+    if (Stream.EmitBlockInfoAbbrev(bitc::FUNCTION_BLOCK_ID, Abbv) !=
+        FUNCTION_DEBUG_LOC_ABBREV)
+      llvm_unreachable("Unexpected abbrev ordering!");
+  }
   Stream.ExitBlock();
 }