[ConstantFold] Support byte values in `bitcast` constant folding (#188030) Add support for constant folding `bitcast` instructions including `ConstantByte` values. This patch handles bitcasts between byte types and integer, FP, and other byte types in both directions. `poison` source bytes are preserved, rather than letting the generic integer fold refine them to `undef` or zero. This is because some threading optimizations just compare the result of constant folding (e.g., https://github.com/llvm/llvm-project/pull/114280). Folds are skipped for byte `bitcast`s where element counts don't divide evenly and the source contains `poison` values. Some of these casts can be folded. However, this is left for a future PR.
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