[IPT] Restructure cache to allow lazy update following invalidation [NFC]

This change restructures the cache used in IPT to point not to the first special instruction, but to the first instruction which *could* be special. That is, the cached reference is always equal to the first special, or comes before it in the block.

This avoids expensive block scans when we are removing special instructions from the beginning of the block. At the moment, this case is not heavily used, though it does trigger in GVN when doing CSE of calls. The main motivation was a change I'm no longer planning to move forward with, but the cache optimization seemed worthwhile as a minor perf win at low cost.

Differential Revision: https://reviews.llvm.org/D111768