[RISCV][TTI] Avoid an infinite recursion issue in getCastInstrCost (#110164) Calling into BasicTTI is not always safe. In particular, BasicTTI does not have a full legalization implementation (vector widening is missing), and falls back on scalarization. The problem is that scalarization for <N x i1> vectors is cost in terms of the cast API and we can end up in an infinite recursive cycle. The "right" fix for this would be teach BasicTTI how to model the full legalization state machine, but several attempts at doing so have resulted in dead ends or undesirable cost changes for targets I don't understand. This patch instead papers over the issue by avoiding the call to the base class when dealing with an i1 source or dest. This doesn't necessarily produce correct costs, but it should at least return something semi-sensible and not crash. Fixes https://github.com/llvm/llvm-project/issues/108708
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