Move `setActiveLaneMask()` from `SpirvShader::EmitState` to `SpirvShader`. Also capitalize the function name, as this is the more common style in this class.

The debugger will want to expose which lanes are active, so we'll need to perform additional work whenever this is called.

This is a no-op change.

Bug: b/145351270
Change-Id: I294c1d7cda08faaa6b784db148b70a7c4d84d431
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39886
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>