glsl2: Add a pass to simplify if statements returning from both sides.

glsl2: Add a pass to simplify if statements returning from both sides.

This allows function inlining making the following tests work even
without function calls implemented:
glsl-fs-functions-2
glsl-fs-functions-3
glsl-vs-functions
glsl-vs-functions-2
glsl-vs-functions-3
glsl-vs-vec4-indexing-5

(Note that those tests were designed to trigger actual function calls,
and this defeats them.  However, those testcases ended up catching the
bug in the previous commit.)