In (Ancient) Quenya, the combinations lgw, rgw eventually became lw, rw, likely passing through lʒw, rʒw in the process. This sound change occurred in both the Ñoldorin and Vanyarin dialects of Quenya as described in the Outline of Phonology (OP2) from the 1950s:

The products of rg, lg; rgw, lgw are in (N.) TQ rr, ll (but rgē̆, lgē̆ > rye, lye); rw, lw; in Vanyarin rg, lg, rw, lw (OP2: PE19/93).

The fact that lgw, rgw > lw, rw was a common change despite the general differences in the Ñoldorin and Vanyarin developments for lg, rg point to it being a comparatively ancient change, in late Ancient Quenya [AQ] or early Parmaquesta [PQ]. Tolkien’s comments later in this document indicate it took place during or before the PQ period:

... especially the fact that even in PQ the product of rgw, lgw is lw, gw, point to the opening of g > ʒ, where it had not become d (OP2: PE19/94).

Tolkien described similarly ancient developments of lgw, rgw [> lʒw, rʒw] > lw, rw in the Outline of Phonetic Development (OP1: PE19/46), but other details of these sound changes were different in the 1940s. See the entry on how spirants became stops after [r], [l] for further discussion.

The only example I’ve found with anything resembling this sound change is:

• ᴹ✶ulgundō > ᴹQ. ulundo “monster” (Ety/ÚLUG).

Here it seems lgu > lʒu > lu, which might be a variation on lgw > lw.