WaveOpsIncludeHelperLanes requirement is too strong

[jeffbolznv]

The WaveOpsIncludeHelperLanes option, specified at https://microsoft.github.io/DirectX-Specs/d3d/HLSL_SM_6_7_Wave_Ops_Include_Helper_Lanes.html, includes this requirement:

For shaders using the new WaveOpsIncludeHelperLanes option, the helper lanes will be guaranteed to exist
until the last wave operation in the shader code, in addition to quad and derivate ops.

On some implementation(s), if all four lanes in a quad are demoted then those invocations will be terminated (immediately exit, no longer running the program). This isn't practical to fix in the implementation, so the requirement should be loosened to match existing behavior.