[ConstantFolding] Fix bitcasting vectors with non-integer ratios (#179640) For both downcast and upcast this patch introduces support for bitcasting vectors with non-integer ratios [1, 2]. Also this patch increases the coherence between casts [3]. 1. Fixes a crash on downcast where the ratio is not an integer (e.g., <3 x i32> to <4 x i24>). Previously this led to leading to an incorrect number of result elements and subsequent crashes in RAUW. Fixes #179626 2. Fixes a miscompilation on upcast where ratio is not an integer (e.g., <4 x i24> to <3 x i32>). Previously this led to wrong result. Fixes #181195 3. Fix incoherent value emitting between down/up cast a. When upcasting, emit Poison and Undef not only if the source vector consists of only one of these values. (e.g., `bitcast <4 x i32> <i32 undef, i32 undef, i32 1, i32 undef> to <2 x i64>` returns `<2 x i64> <i64 undef, i64 1>` b. When downcasting, when SrcElt is Poison, handle it as Poison and not Undef. (e.g., `bitcast <1 x i32> <i32 poison> to <2 x i16>` returns `<2 x i16> poison`
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