[LV] Add target-independent test for type-mismatch

The test suite of LoopVectorize suffers from a coverage hole when types
mismatch, in the target-independent case. There is already
interleave-allocsize-not-equal-typesize.ll under the AArch64 target
written for the purposes of fixing specific bugs, but nothing exercising
this in a target-independent fashion. Fix this by adapting a test from
LoopAccessAnalysis' depend_diff_types.ll, and demonstrate that
LoopVectorize only ever interleaves when types mismatch.

Author: artagnon