JIT: have inliner use fgSplitBlockAfterStatement (#80928)

In anticipation of moving pred list building before inlining,
use the common block split utility instead of custom code. This does a
better job of propagating IL offsets, and automatically handles
pred list maintenance.

Contributes to #80193.

src/coreclr/jit/fginline.cpp

@@ -1164,78 +1164,7 @@ void Compiler::fgInsertInlineeBlocks(InlineInfo* pInlineInfo)
     //
     // ======= Inserting inlinee's basic blocks ===============
     //
-
-    bottomBlock             = fgNewBBafter(topBlock->bbJumpKind, topBlock, true);
-    bottomBlock->bbRefs     = 1;
-    bottomBlock->bbJumpDest = topBlock->bbJumpDest;
-    bottomBlock->inheritWeight(topBlock);
-
-    topBlock->bbJumpKind = BBJ_NONE;
-
-    // Update block flags
-    {
-        const BasicBlockFlags originalFlags = topBlock->bbFlags;
-        noway_assert((originalFlags & BBF_SPLIT_NONEXIST) == 0);
-        topBlock->bbFlags &= ~(BBF_SPLIT_LOST);
-        bottomBlock->bbFlags |= originalFlags & BBF_SPLIT_GAINED;
-    }
-
-    // Split statements between topBlock and bottomBlock.
-    // First figure out bottomBlock_Begin
-    Statement* bottomBlock_Begin;
-    bottomBlock_Begin = stmtAfter->GetNextStmt();
-
-    if (topBlock->bbStmtList == nullptr)
-    {
-        // topBlock is empty before the split.
-        // In this case, both topBlock and bottomBlock should be empty
-        noway_assert(bottomBlock_Begin == nullptr);
-        topBlock->bbStmtList    = nullptr;
-        bottomBlock->bbStmtList = nullptr;
-    }
-    else if (topBlock->bbStmtList == bottomBlock_Begin)
-    {
-        noway_assert(bottomBlock_Begin != nullptr);
-
-        // topBlock contains at least one statement before the split.
-        // And the split is before the first statement.
-        // In this case, topBlock should be empty, and everything else should be moved to the bottomBlock.
-        bottomBlock->bbStmtList = topBlock->bbStmtList;
-        topBlock->bbStmtList    = nullptr;
-    }
-    else if (bottomBlock_Begin == nullptr)
-    {
-        noway_assert(topBlock->bbStmtList != nullptr);
-
-        // topBlock contains at least one statement before the split.
-        // And the split is at the end of the topBlock.
-        // In this case, everything should be kept in the topBlock, and the bottomBlock should be empty
-
-        bottomBlock->bbStmtList = nullptr;
-    }
-    else
-    {
-        noway_assert(topBlock->bbStmtList != nullptr);
-        noway_assert(bottomBlock_Begin != nullptr);
-
-        // This is the normal case where both blocks should contain at least one statement.
-        Statement* topBlock_Begin = topBlock->firstStmt();
-        noway_assert(topBlock_Begin != nullptr);
-        Statement* topBlock_End = bottomBlock_Begin->GetPrevStmt();
-        noway_assert(topBlock_End != nullptr);
-        Statement* bottomBlock_End = topBlock->lastStmt();
-        noway_assert(bottomBlock_End != nullptr);
-
-        // Break the linkage between 2 blocks.
-        topBlock_End->SetNextStmt(nullptr);
-
-        // Fix up all the pointers.
-        topBlock->bbStmtList = topBlock_Begin;
-        topBlock->bbStmtList->SetPrevStmt(topBlock_End);
-
-        bottomBlock->bbStmtList = bottomBlock_Begin;
-        bottomBlock->bbStmtList->SetPrevStmt(bottomBlock_End);
-    }
+    bottomBlock = fgSplitBlockAfterStatement(topBlock, stmtAfter);
 
     //
     // Set the try and handler index and fix the jump types of inlinee's blocks.