[flang][NFC] AliasAnalysis: Use Indirect not Unknown for LoadOp

As mentioned at
<#126156 (comment)>:
PR #126156 causes AliasAnalysis::getSource to sometimes return
SourceKind::Unknown when it used to return SourceKind::Indirect for a
LoadOp.  This patch restores that part of the old behavior.  It should
not affect user-visible behavior because AliasAnalysis::alias treats
SourceKind::Unknown and SourceKind::Indirect equivalently, but it does
improve debugging output.

Author: jdenny-ornl

flang/lib/Optimizer/Analysis/AliasAnalysis.cpp

@@ -641,14 +641,13 @@ AliasAnalysis::Source AliasAnalysis::getSource(mlir::Value v,
             } else if (isDummyArgument(def)) {
               defOp = nullptr;
               v = def;
+            } else {
+              type = SourceKind::Indirect;
             }
-
-            breakFromLoop = true;
-            return;
+          } else {
+            // No further tracking for addresses loaded from memory for now.
+            type = SourceKind::Indirect;
           }
-
-          // No further tracking for addresses loaded from memory for now.
-          type = SourceKind::Indirect;
           breakFromLoop = true;
         })
         .Case<fir::AddrOfOp>([&](auto op) {