[ARM GlobalISel] Update legalizer test

Make one of the legalizer tests a bit more robust by making sure all
values we're interested in are used (either in a store or a return) and
by using loads instead of constants for obtaining values on fewer than
32 bits. This should make the test less fragile to changes in the
legalize combiner, since those loads are legal (as opposed to the
constants, which were being widened and thus produced opportunities for
the legalize combiner).

llvm-svn: 318047