[AMDGPU][SIPreEmitPeephole] Missing condition in mustRetainExeczBranch

[jmmartinez]

If the code in the "then" block is modifying the exec mask, we must retain the s_cbranch_execz branch.

Consider this example:

    s_cbranch_execz after
    s_or_b32 exec_lo, exec_lo, -1
after:
    ...

If the branch is removed, when we reach after exec is never zero, while before it would have been zero.

I stumbled upon this bug by accident. I'm trying to see if the bug is more general than this (this should be a problem for any SALU operation writing to a non-SSA register) and how to test it. Maybe something like

    bool DontMoveAcrossStore = true;
    if (??? && !MI.isSafeToMove(DontMoveAcrossStore))
      return true;

Edit: I saw an example where there was a mov to m0

 ; GFX8-NEXT:    s_xor_b64 s[0:1], exec, s[0:1]
+; GFX8-NEXT:    s_cbranch_execz .LBB29_8
 ; GFX8-NEXT:  ; %bb.7:
 ; GFX8-NEXT:    v_mov_b32_e32 v2, s2
 ; GFX8-NEXT:    s_mov_b32 m0, -1
 ; GFX8-NEXT:    ds_add_rtn_f32 v2, v2, v1
-; GFX8-NEXT:  ; %bb.8:
+; GFX8-NEXT:  .LBB29_8:

Edit++: I just realized that this pass is run after the virtual register are lowered to physical registers, so any scalar operation writing to a physical register would pose a problem. Am I right ?