glsl2: Catch pointless copies in copy propagation.

glsl2: Catch pointless copies in copy propagation.

We wouldn't want to go rewriting dereferences to variables to point at
the same variable it did before.  While I didn't find a way to trigger
that, a shader in Yo Frankie managed to produce a self-assignment by
passing a constant to a function doing self assignment like this.

Cleans up the IR for glsl-deadcode-self-assign.shader_test