[Polly][CodeGen] Allow nesting of BandAttr mark without loop.

BandAttr markers are added as parents of schedule tree bands. These also
appear as markers its equivalent AST, but a band does not necessarily
corresponds to a loop in this. Iterations may be peeled or the loop
being unrolled (e.g. if it has just one iteration). In such cases it may
happend that there is not loop between a BandAttr marker and the marker
for a loop nested in the former parent band/loop.

Handle the situation by giving priority to the inner marker over the
outer.

Fixes the polly-x86_64-linux-test-suite buildbot.