[LAA] Only invalidate loops that require runtime checks (NFCI).

LAA doesn't keep references to IR outside the loop or references to
SCEVs that may be invalidated, unless runtime checks are needed (either
memory or SCEV predicates). For the current LAA users, it should be
sufficient to invalidate entries for loops that require runtime checks,
thus avoiding analyzing loops again unnecessarily.

This helps reduce compile-time, in particular when removing the
restrictions added in 234cc40.

https://llvm-compile-time-tracker.com/compare.php?from=73894dba2cdbcc00678d0c13a6b61765675f60b4&to=05c6bdc41b5f63696ebeb7116325725fa94f66d6&stat=instructions:u

Author: fhahn

llvm/include/llvm/Analysis/LoopAccessAnalysis.h

@@ -863,7 +863,7 @@ class LoopAccessInfoManager {
 
   const LoopAccessInfo &getInfo(Loop &L);
 
-  void clear() { LoopAccessInfoMap.clear(); }
+  void clear();
 
   bool invalidate(Function &F, const PreservedAnalyses &PA,
                   FunctionAnalysisManager::Invalidator &Inv);

llvm/lib/Analysis/LoopAccessAnalysis.cpp

@@ -3084,6 +3084,22 @@ const LoopAccessInfo &LoopAccessInfoManager::getInfo(Loop &L) {
 
   return *It->second;
 }
+void LoopAccessInfoManager::clear() {
+  SmallVector<Loop *> ToRemove;
+  // Collect LoopAccessInfo entries that may keep references to IR outside the
+  // analyzed loop or SCEVs that may have been modified or invalidated. At the
+  // moment, that is loops requiring memory or SCEV runtime checks, as those cache
+  // SCEVs, e.g. for pointer expressions.
+  for (const auto &[L, LAI] : LoopAccessInfoMap) {
+    if (LAI->getRuntimePointerChecking()->getChecks().empty() &&
+        LAI->getPSE().getPredicate().isAlwaysTrue())
+      continue;
+    ToRemove.push_back(L);
+  }
+
+  for (Loop *L : ToRemove)
+    LoopAccessInfoMap.erase(L);
+}
 
 bool LoopAccessInfoManager::invalidate(
     Function &F, const PreservedAnalyses &PA,