JIT: Unprotect handler entry after finally removal

[amanasifkhalid]

Found while working on #115850. Suppose we have the following flowgraph:

---------------------------------------------------------------------------------------------------------------------------------------------------------------------
BBnum BBid ref try hnd preds           weight   [IL range]   [jump]                            [EH region]        [flags]
---------------------------------------------------------------------------------------------------------------------------------------------------------------------
BB01 [0000]  1                             1    [000..00D)-> BB02(1)                 (always)                   i
BB02 [0001]  1  0    BB01                  1    [00D..00F)-> BB03(1)                 (callf ) T0    try {       i keep
BB07 [0006]  1  0    BB05                  1    [???..???)-> BB08(1)                 (callfr) T0    }           i internal
BB08 [0007]  1       BB07                  1    [???..???)-> BB06(1)                 (ALWAYS)                   i internal KEEP
BB03 [0002]  2     0 BB02                  1    [00F..012)-> BB05(0.5),BB04(0.5)     ( cond )    H0 finally {   i keep
BB04 [0003]  1     0 BB03                  1    [012..018)-> BB05(1)                 (always)    H0             i
BB05 [0004]  2     0 BB03,BB04             1    [018..019)-> BB07(1)                 (finret)    H0 }           i
BB06 [0005]  1       BB08                  1    [019..01A)                           (return)                   i
---------------------------------------------------------------------------------------------------------------------------------------------------------------------

fgRemoveEmptyTry will remove the BBF_DONT_REMOVE guard from the try entry if it decides to remove the region, but it won't do so for the corresponding finally entry:

---------------------------------------------------------------------------------------------------------------------------------------------------------------------
BBnum BBid ref try hnd preds           weight   [IL range]   [jump]                            [EH region]        [flags]
---------------------------------------------------------------------------------------------------------------------------------------------------------------------
BB01 [0000]  1                             1    [000..00D)-> BB02(1)                 (always)                   i
BB02 [0001]  1       BB01                  1    [00D..00F)-> BB03(1)                 (always)                   i
BB08 [0007]  1       BB05                  1    [???..???)-> BB06(1)                 (always)                   i internal
BB03 [0002]  1       BB02                  1    [00F..012)-> BB05(0.5),BB04(0.5)     ( cond )                   i keep
BB04 [0003]  1       BB03                  1    [012..018)-> BB05(1)                 (always)                   i
BB05 [0004]  2       BB03,BB04             1    [018..019)-> BB08(1)                 (always)                   i
BB06 [0005]  1       BB08                  1    [019..01A)                           (return)                   i
---------------------------------------------------------------------------------------------------------------------------------------------------------------------

If we remove the guard for the finally entry in this case, loop inversion will make it unreachable, and flow opts will remove it entirely, rather than converting it into an unnecessary BBJ_THROW. This change enables that.

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Pull Request Overview

This PR ensures that the handler entry block loses its BBF_DONT_REMOVE guard when an empty try is removed, and it cleans up duplicate flag removals in the cloned finally logic.

• Unprotect handler entry block in fgRemoveEmptyTry
• Consolidate BBF_DONT_REMOVE removals in fgCloneFinally

[amanasifkhalid]

@dotnet/jit-contrib PTAL. Small diffs. Thanks!

[amanasifkhalid]

/ba-g android build stuck