spirv: Clamp ftou output.

SPIR-V says this is undefined behaviour, but Direct3D actually specifies that it should clamp. Most drivers do clamp, but llvmpipe does not. Hence this fixes a couple of tests with llvmpipe.

This does of course add overhead to the ftou instruction, but I cannot imagine that it is a common instruction to execute. This also is not the first time we perform such checks; cf. udiv.