glsl/loops: Allocate loop_terminator using new(mem_ctx) syntax.

Patches to follow will introduce code into the loop_terminator
constructor.  Allocating loop_terminator using new(mem_ctx) syntax
will ensure that the constructor runs.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/loop_analysis.cpp

@@ -134,7 +134,7 @@ loop_terminator *
 loop_variable_state::insert(ir_if *if_stmt)
 {
    void *mem_ctx = ralloc_parent(this);
-   loop_terminator *t = rzalloc(mem_ctx, loop_terminator);
+   loop_terminator *t = new(mem_ctx) loop_terminator();
 
    t->ir = if_stmt;
    this->terminators.push_tail(t);