Only initialize through a temporary variable if the same symbol name is used in the initialization expression.
TRAC #13627
The previous patch can generate a lot of unnecessary temporary variables. By first checking whether the same symbol name is reused the clutter is reduced to an absolute minimum (typical shaders won't rely on this odd GLSL semantic behavior so the workaround is hardly ever needed).
Signed-off-by: Daniel Koch

Author:    Nicolas Capens

git-svn-id: https://angleproject.googlecode.com/svn/trunk@479 736b8ea6-26fd-11df-bfd4-992fa37f6226