AMDGPU: Fix a corner case crash in SIOptimizeExecMasking

Summary:
See the new test case; this is really unlikely to happen with real code,
but I ran into this while attempting to bugpoint-reduce a different issue.

Change-Id: I9ade1dc1aa8fd9c4d9fc83661d7b80e310b5c4a6

Reviewers: arsenm, rampitec

Subscribers: kzhuravl, wdng, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D45885

llvm-svn: 330585