[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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