Fix getCommonType in a different way from the way I fixed it when

working on FCA splitting. Instead of refusing to form a common type when
there are uses of a subsection of the alloca as well as a use of the
entire alloca, just skip the subsection uses and continue looking for
a whole-alloca use with a type that we can use.

This produces slightly prettier IR I think, and also fixes the other
failure in the test.

llvm-svn: 164146