[x86] Add some important, missing test coverage for blending from one
vector to a zero vector for the v2 cases and fix the v4 integer cases to
actually blend from a vector.

There are already seprate tests for the case of inserting from a scalar.

These cases cover a lot of the regressions I've seen in the regression
test suite for the new vector shuffle lowering and specifically cover
the reported lack of using various zext-ing instruction patterns. My
next patch should fix a big chunk of this, but wanted to get a nice
baseline for these patterns in the test cases first.