Fix accidentally constant conditions found by uncommitted improvements to -Wconstant...

Fix accidentally constant conditions found by uncommitted improvements to -Wconstant-conversion.

A couple of cases where we were accidentally creating constant conditions by
something like "x == a || b" instead of "x == a || x == b". In one case a
conditional & then unreachable was used - I transformed this into a direct
assert instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154324 91177308-0d34-0410-b5e6-96231b3b80d8