AMDGPU: Fix use-after-free in SIOptimizeExecMasking

Summary:
There was a bug with sequences like

   s_mov_b64 s[0:1], exec
   s_and_b64 s[2:3]<def>, s[0:1], s[2:3]<kill>
   ...
   s_mov_b64_term exec, s[2:3]

because s[2:3] was defined and used in the same instruction, ending up with
SaveExecInst inside OtherUseInsts.

Note that the test case also exposes an unrelated bug.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98028

Reviewers: tstellarAMD, arsenm

Subscribers: kzhuravl, wdng, yaxunl, llvm-commits, tony-tye

Differential Revision: https://reviews.llvm.org/D25306

llvm-svn: 283528