Subzero emits the following sequence when extracting elements from a
vector of i1:

vmov.8 Rt, Dm[I]

I should be scaled when accessing v4i1, and v8i1, i.e., to extract the
n-th boolean in a v8i1, the emitted code should be

vmov.8 Rt, Dm[I*n]

Insertions are handled by changing the operands' types, so that a
v4i1 is handled as a v4i32, and a v8i1, as a v8i16. I.e., to insert
the n-th boolean into a v8i1, the emitted code should be

mov.16 Dt[I], Rm

instead of

mov.8 Dt[I*n], Rm

This clears the upper bits for that element.

BUG=
R=eholk@chromium.org

Review URL: https://codereview.chromium.org/1876083004 .