glsl: stop copying struct and interface member names

We are currently copying the name for each member dereference
but we can just share a single instance of the string provided
by the type.

This change also stops us recalculating the field index
repeatedly.

Reviewed-by: Thomas Helland <thomashelland90@gmail.com>

src/compiler/glsl/ir.h

@@ -2108,7 +2108,7 @@ public:
    virtual ir_visitor_status accept(ir_hierarchical_visitor *);
 
    ir_rvalue *record;
-   const char *field;
+   int field_idx;
 };
 
 
@@ -2192,7 +2192,7 @@ public:
 
    ir_constant *get_array_element(unsigned i) const;
 
-   ir_constant *get_record_field(const char *name);
+   ir_constant *get_record_field(int idx);
 
    /**
     * Copy the values on another constant at a given offset.