AMDGPU: Fix a SIAnnotateControlFlow issue when there are multiple backedges.

Summary:
At the exit of the loop, the compiler uses a register to remember and accumulate
the number of threads that have already exited. When all active threads exit the
loop, this register is used to restore the exec mask, and the execution continues
for the post loop code.

When there is a "continue" in the loop, the compiler made a mistake to reset the
register to 0 when the "continue" backedge is taken. This will result in some
threads not executing the post loop code as they are supposed to.

This patch fixed the issue.

Reviewers:
  nhaehnle, arsenm

Differential Revision:
  https://reviews.llvm.org/D59312

llvm-svn: 356298