This was initially made in 681973c3.
Here I wanted to enforce that the alignment passed to %memcpy was a
constant expression, as this is required by LLVM. However, this breaks
the knot-tying done in `loopDecls`, causing T8131 to hang.

Here I remove the `seq` and mark T8131 as `expect_broken` in the case
of the NCG, which doesn't force the alignment in this case.

Fixes #10664.