glsl: Fix regression in function out-parameter lvalue detection.

glsl: Fix regression in function out-parameter lvalue detection.

When doing the var->assigned change in
f2475ca424f7e001be50f64dafa5700f6603d684, I overzealously indented the
second block of code into the "if (var)" test.  Revert these blocks to
the way they were before, just taking advantage of "var" to avoid
re-calling variable_referenced().

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=49066