All switch control flows return, but the compiler doesn't seem smart enough to notice this.

This magically started happening when I switched to using ninja, possibly suggesting that it is either suddenly using a different compiler or the flags are different.

While something I'll try investigating, here's a fix all the same.

Bug: b/130335507
Change-Id: I3c6d00c00ecd03bbd34d51ac62d02fefdc026526
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/32649
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>