[Instcombine] Avoid widening trunc+sext to trunc+shl+ashr when not profitable (#160483) Skip the transform that replaces: %a = trunc i64 %x to i16 %b = sext i16 %a to i32 with %a = trunc i64 %x to i32 %b = shl i32 %a, 16 %c = ashr exact i32 %b, 16 when (pre-trunc) source type is wider than the final destination type. Modern architectures typically have efficient sign-extend instruction. It is preferable to preserve the sext for this case. Resolves #116019 Also fold sext(trunc nsw) to trunc nsw when narrowing source.
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