non-asserts builders are failing since `vecTy` is unused aside from this
one assertion: https://lab.llvm.org/buildbot/#/builders/228/builds/4754
Since side-effects here are uninteresting, move the entire expr into the
assert, per CodingStandards.md
Fix-forward for #199700
(cherry picked from commit e217dcb8de5dd83b307fe8b95b43de11a7d894ab)