Cleanup and fix an assert that was mis-firing.

Note that there is no test suite update. This was found by a couple of
tests failing when the test suite was run on a powerpc64 host (thanks
Roman!). The tests don't specify a triple, which might seem surprising
for a codegen test. But in fact, these tests don't even inspect their
output. Not at all. I could add a bunch of triples to these tests so
that we'd get the test coverage for normal builds, but really someone
needs to go through and add actual *tests* to these tests. =[ The ones
in question are:

  test/CodeGen/bitfield-init.c
  test/CodeGen/union.c

llvm-svn: 169694