glsl: Fix variable_referenced() for vector_{extract,insert} expressions

glsl: Fix variable_referenced() for vector_{extract,insert} expressions

We get these when we operate on vector variables with array accessors
(i.e. things like a[0] where 'a' is a vec4). When we call variable_referenced()
on these expressions we want to return a reference to 'a' instead of NULL.

This fixes a problem where we pass a[0] as the first argument to an atomic
SSBO function that expects a buffer variable. In order to check this, we use
variable_referenced(), but that is currently returning NULL in this case, since
the underlying rvalue is a vector_extract expression.

Tested-by: Markus Wick <markus@selfnet.de>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>