AMDGPU: Skip other terminators before inserting s_cbranch_exec[n]z

PHIElimination/createPHISourceCopy inserts non-branch terminators
after the control flow pseudo if a successor phi reads a register
defined by the control flow pseudo. If this happens, we need to split
the expansion of the control flow pseudo to ensure all the branches
are after all of the other mask management instructions.

GlobalISel hit this in testscases that happened to be tail
duplicated. The original testcase still does not work, since the same
problem appears to be present in a later pass.