Missing loop vectorization due to optimization in InstrCombine

[thevar1able]

Loop vectorization that worked with clang-19 stopped with clang-20.

Minimal repro:
https://godbolt.org/z/Yd4KjG9Yo
https://godbolt.org/z/cP5jeeo5Y

I bisected the regression to this PR. Looks like wrapping inside select(...) confuses LoopVectorizer.

Backreference: ClickHouse/ClickHouse#77352 (comment)