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`
2ce48b2691