Fix a crash in DwarfDebug::validThroughout.

The instruction it falls over on is an IMPLICT_DEF that also happens
to be the only instruction in its lexical scope. That LexicalScope has
never been created because its range is empty. This patch skips over
all meta-instructions instead of just DBG_VALUEs.

Thanks to David Blaikie for providing a testcase!

llvm-svn: 305853