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

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