JIT: Allow spill-at-single-def for pure defs (#85251)

Allow the spill-at-single-def logic to kick in for defs without subsequent uses before the spill.

src/coreclr/jit/lsra.cpp

@@ -3423,15 +3423,11 @@ void LinearScan::spillInterval(Interval* interval, RefPosition* fromRefPosition
         }
     }
 
-    // Only handle the singledef intervals whose firstRefPosition is RefTypeDef and is not yet marked as spillAfter.
-    // The singledef intervals whose firstRefPositions are already marked as spillAfter, no need to mark them as
-    // singleDefSpill because they will always get spilled at firstRefPosition.
-    // This helps in spilling the singleDef at definition
+    // Only handle the singledef intervals whose firstRefPosition is RefTypeDef.
     //
     // Note: Only mark "singleDefSpill" for those intervals who ever get spilled. The intervals that are never spilled
     // will not be marked as "singleDefSpill" and hence won't get spilled at the first definition.
-    if (interval->isSingleDef && RefTypeIsDef(interval->firstRefPosition->refType) &&
-        !interval->firstRefPosition->spillAfter)
+    if (interval->isSingleDef && RefTypeIsDef(interval->firstRefPosition->refType))
     {
         // TODO-CQ: Check if it is beneficial to spill at def, meaning, if it is a hot block don't worry about
         // doing the spill. Another option is to track number of refpositions and a interval has more than X
@@ -6280,6 +6276,11 @@ void LinearScan::resolveLocalRef(BasicBlock* block, GenTreeLclVar* treeNode, Ref
         varDsc->SetRegNum(REG_STK);
         interval->physReg = REG_NA;
         writeLocalReg(treeNode->AsLclVar(), interval->varNum, REG_NA);
+
+        if (currentRefPosition->singleDefSpill)
+        {
+            varDsc->lvSpillAtSingleDef = true;
+        }
     }
     else // Not reload and Not pure-def that's spillAfter
     {