glsl2: Add a pass for removing unused functions.

glsl2: Add a pass for removing unused functions.

For a shader involving many small functions, this avoids running
optimization across all of them after they've been inlined
post-linking.

Reduces the runtime of linking and running a fragment shader from Yo
Frankie from 1.6 seconds to 0.9 seconds (-44.9%, +/- 3.3%).