Reuse the previous instruction's scope for line 0 locations

for easier-to-read debug info.

This has no practical effect otherwise.

Author: adrian-prantl

lib/IRGen/IRGenSIL.cpp

@@ -1693,9 +1693,14 @@ void IRGenSILFunction::visitSILBasicBlock(SILBasicBlock *BB) {
       // Set the builder's debug location.
       if (DS && !KeepCurrentLocation)
         IGM.DebugInfo->setCurrentLoc(Builder, DS, ILoc);
-      else
+      else {
+        // Reuse the last scope for an easier-to-read line table.
+        auto Prev = --I.getIterator();
+        if (Prev != BB->end())
+          DS = Prev->getDebugScope();
         // Use an artificial (line 0) location.
         IGM.DebugInfo->setCurrentLoc(Builder, DS);
+      }
 
       if (isa<TermInst>(&I))
         emitDebugVariableRangeExtension(BB);