In MVE, as of rL371218, we are attempting to sink chains of instructions such as:
%l1 = insertelement <8 x i8> undef, i8 %l0, i32 0 %broadcast.splat26 = shufflevector <8 x i8> %l1, <8 x i8> undef, <8 x i32> zeroinitializer
In certain situations though, we can end up breaking the dominance relations of instructions. This happens when we sink the instruction into a loop, but cannot remove the originals. The Use is updated, which might in fact be a Use from the second instruction to the first.
This attempts to fix that by reversing the order of instruction that are sunk, and ensuring that we update the uses on new instructions if they have already been sunk, not the old ones.