Fix GC Poll inlining. (dotnet#39881)

* Don't inline GC polls in cold basic blocks.
* Allow GC poll inlining in basic blocks with `BBF_LOOP_PREHEADER` or `BBF_RETLESS_CALL` set.

This fixes one of the assert seen in dotnet#39474 (comment)

Contributes to resolving dotnet#39726.

src/coreclr/src/jit/flowgraph.cpp

@@ -3699,6 +3699,18 @@ PhaseStatus Compiler::fgInsertGCPolls()
             // We don't want to deal with all the outgoing edges of a switch block.
             pollType = GCPOLL_CALL;
         }
+        else if ((block->bbFlags & BBF_COLD) != 0)
+        {
+#ifdef DEBUG
+            if (verbose)
+            {
+                printf("Selecting CALL poll in block " FMT_BB " because it is a cold block\n", block->bbNum);
+            }
+#endif // DEBUG
+
+            // We don't want to split a cold block.
+            pollType = GCPOLL_CALL;
+        }
 
         BasicBlock* curBasicBlock = fgCreateGCPoll(pollType, block);
         createdPollBlocks |= (block != curBasicBlock);
@@ -4141,9 +4153,12 @@ BasicBlock* Compiler::fgCreateGCPoll(GCPollType pollType, BasicBlock* block)
 
         // We are allowed to split loops and we need to keep a few other flags...
         //
-        noway_assert((originalFlags & (BBF_SPLIT_NONEXIST & ~(BBF_LOOP_HEAD | BBF_LOOP_CALL0 | BBF_LOOP_CALL1))) == 0);
-        top->bbFlags = originalFlags & (~BBF_SPLIT_LOST | BBF_GC_SAFE_POINT);
-        bottom->bbFlags |= originalFlags & (BBF_SPLIT_GAINED | BBF_IMPORTED | BBF_GC_SAFE_POINT);
+        noway_assert((originalFlags & (BBF_SPLIT_NONEXIST &
+                                       ~(BBF_LOOP_HEAD | BBF_LOOP_CALL0 | BBF_LOOP_CALL1 | BBF_LOOP_PREHEADER |
+                                         BBF_RETLESS_CALL))) == 0);
+        top->bbFlags = originalFlags & (~(BBF_SPLIT_LOST | BBF_LOOP_PREHEADER | BBF_RETLESS_CALL) | BBF_GC_SAFE_POINT);
+        bottom->bbFlags |= originalFlags & (BBF_SPLIT_GAINED | BBF_IMPORTED | BBF_GC_SAFE_POINT | BBF_LOOP_PREHEADER |
+                                            BBF_RETLESS_CALL);
         bottom->inheritWeight(top);
         poll->bbFlags |= originalFlags & (BBF_SPLIT_GAINED | BBF_IMPORTED | BBF_GC_SAFE_POINT);