[VectorCombine] Fold reduce(trunc(x)) -> trunc(reduce(x)) iff cost effective (#81852)

Vector truncations can be pretty expensive, especially on X86, whilst scalar truncations are often free.

If the cost of performing the add/mul/and/or/xor reduction is cheap enough on the pre-truncated type, then avoid the vector truncation entirely.

Fixes https://github.com/llvm/llvm-project/issues/81469