glsl: Free the loop state context when we free the loop state.

Since this was talloced off of NULL instead of the compile state, it
was a real leak over the course of the program.  Noticed with
valgrind --leak-check=full --show-reachable=yes.  We should really
change these passes to generally get the compile context as an argument
so simple mistakes like this stop mattering.
(cherry picked from commit 6929cdd14b)

src/glsl/loop_analysis.cpp

@@ -44,6 +44,7 @@ loop_state::loop_state()
 loop_state::~loop_state()
 {
    hash_table_dtor(this->ht);
+   talloc_free(this->mem_ctx);
 }