Tolkien said that rl and lr became ll in both the Outline of Phonetic Development (OP1) from the 1930s and the Outline of Phonology (OP2) from the 1950s:

ll, lr, rl: These all gave phonetically ll; but lar, r-l occurred by grammatical analogy (OP1: PE19/49).

ll remained, and was a greatly favoured combination; rl and lr also became ll. In the case of rl (not lr) later reformations produced r-l with intruded vowel determined by the quality of the vowel preceding the r (OP2: PE19/100).

It is possible that the rule whereby [lr] became [ll] was actually a phonetic development in Primitive Elvish, since there is evidence of this phonetic change in all Eldarin language branches in The Etymologies of the 1930s:

• ᴹ✶kalrō > ᴹQ. kallo “hero”; ᴹ✶kalrondō > N. callon (Ety/KAL).
• ᴹ✶stalrā > Ilk. thall “steep” (Ety/STAL).
• ᴹ✶talrunya > ᴹQ. tallune, N. tellen “sole of foot” (Ety/RUN; Ety/TAL).

There is likewise evidence that rl > ll in (Old) Sindarin:

• OS. glawar-lin > S. Glewellin “Song of Gold” (PE17/61).

However, it is possible these were parallel sound changes rather than occurring during the Primitive Elvish period, as suggested by David Salo (GS/§4.160, §4.161). Nevertheless these sound change seem to be ancient and no longer active in Sindarin. The combinations rl and lr are fairly common in Sindarin/Noldorin compounds, and rl appears in at least one Quenya compound: ᴹQ. Erlaire “June”, probably er + laire “*first summer” and thus it is likely an Exilic compound.

Tolkien indicated there were assimilations dl, zl > ll later in Quenya’s phonetic history, and since both d and z became r in the Ñoldorin dialect of Quenya, these might represent another later example of rl > ll. However, since these changes occurred in Vanyarin as well, more likely these assimilations began before d, z > r: see the entry on how [z] and [ð] assimilated to following [r], [l] for further details.

On the other hand, it’s quite likely rl > ll remained an active phonetic rule in Quenya for a long time, probably even into Tarquesta [TQ]. For example, r + l > ll in the partitive plural, as in Kasalli the partitive plural form of Q. Kasar “Dwarf” (WJ/388).

Conceptual Development: I have found no direct evidence of either lr or rl > ll as an ancient sound change in the Early Quenya of the 1910s and 1920s, but Tolkien did indicate rl > ll (or zl > ll) as an assimilation occurring after other sound changes:

[Medial] s gave z and then r before l, r, m, n, w̑, y̑, ƀ, ʒ, giving ll, ´r, rn, rm, rw, ry, rb [or] rw, rg [or] ´r (PE12/19).