AMDGPU: Add additional MIR tests for exec mask optimizations

AMDGPU: Add additional MIR tests for exec mask optimizations

Also includes one example of how this transform is unsound. This isn't
verifying the copies are used in the control flow intrinisic patterns.

Also add option to disable exec mask opt pass. Since this pass is
unsound, it may be useful to turn it off until it is fixed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@357091 91177308-0d34-0410-b5e6-96231b3b80d8