JIT: Propagate GC safe point flag in fgSplitBlockAtBeginning (#116718)

Fixes #116704. From that failure in particular, we have the following flowgraph before loop inversion runs:

```
---------------------------------------------------------------------------------------------------------------------------------------------------------------------
BBnum BBid ref try hnd preds           weight   [IL range]   [jump]                            [EH region]        [flags]
---------------------------------------------------------------------------------------------------------------------------------------------------------------------
BB01 [0000]  1                             1    [000..004)-> BB03(0.5),BB02(0.5)     ( cond )                   i
BB02 [0001]  1       BB01                  1    [004..005)                           (return)                   i
BB03 [0002]  1       BB01                  1    [005..04E)-> BB05(1)                 (always)                   i hascall gcsafe newobj
BB04 [0003]  1       BB05                  1    [04E..063)-> BB05(1)                 (always)                   i hascall gcsafe idxlen bwd bwd-target
BB05 [0004]  2       BB03,BB04             1    [063..06A)-> BB04(0.5),BB06(0.5)     ( cond )                   i bwd bwd-src
BB06 [0005]  1       BB05                  1    [06A..0B9)                           (throw )                   i hascall gcsafe newobj
---------------------------------------------------------------------------------------------------------------------------------------------------------------------
```

`BB06` has a tail call, and is flagged as having a GC safe point. It is also the exit block of a loop headed by `BB05`. Loop inversion gives `BB06` an empty predecessor to keep the loop canonical by calling `fgSplitBlockAtBeginning`. This calls `fgSplitBlockAtEnd`, which removes the safe point flag on the new block since it initially keeps the code in the predecessor. `fgSplitBlockAtBeginning` does not reset the flag after moving the code into the new successor. Thus, we end up with the following flowgraph:

```
---------------------------------------------------------------------------------------------------------------------------------------------------------------------
BBnum BBid ref try hnd preds           weight   [IL range]   [jump]                            [EH region]        [flags]
---------------------------------------------------------------------------------------------------------------------------------------------------------------------
BB01 [0000]  1                             1    [000..004)-> BB03(0.5),BB02(0.5)     ( cond )                   i
BB02 [0001]  1       BB01                  1    [004..005)                           (return)                   i
BB03 [0002]  1       BB01                  1    [005..04E)-> BB09(0.5),BB08(0.5)     ( cond )                   i hascall gcsafe newobj
BB08 [0021]  1       BB03                  1    [???..???)-> BB04(1)                 (always)                   i hascall newobj
BB04 [0003]  2       BB04,BB08             1    [04E..06A)-> BB04(0.5),BB06(0.5)     ( cond )                   i hascall gcsafe idxlen bwd bwd-target
BB06 [0005]  1       BB04                  1    [???..???)-> BB09(1)                 (always)                   i hascall gcsafe newobj
BB09 [0022]  2       BB03,BB06             1    [06A..0B9)                           (throw )                   i hascall newobj
---------------------------------------------------------------------------------------------------------------------------------------------------------------------
```

Note the presence of the flag on `BB06`, which has no IR, and the lack of the flag on `BB09`, which contains the actual tail call.

Author: amanasifkhalid

src/coreclr/jit/fgbasic.cpp

@@ -4922,6 +4922,12 @@ BasicBlock* Compiler::fgSplitBlockAtBeginning(BasicBlock* curr)
     curr->bbCodeOffs    = BAD_IL_OFFSET;
     curr->bbCodeOffsEnd = BAD_IL_OFFSET;
 
+    // fgSplitBlockAtEnd conservatively removes the BBF_GC_SAFE_POINT flag from the new block.
+    // Since we moved all of the code back into the new block, set the flag again, if applicable.
+    // For the same reason as fgSplitBlockAtEnd, we will conservatively remove the flag from 'curr'.
+    newBlock->CopyFlags(curr, BBF_GC_SAFE_POINT);
+    curr->RemoveFlags(BBF_GC_SAFE_POINT);
+
     return newBlock;
 }