[AsmPrinter] Collapse .loc 0 0 directives

Currently we do not always collapse subsequent .loc 0 0 directives. The
reason is that we were checking for a PrevInstLoc which is not set when
we emit a line-0 record. We should only check the LastAsmLine, which
seems to be created exactly for this purpose.

  // When we emit a line-0 record, we don't update PrevInstLoc; so look at
  // the last line number actually emitted, to see if it was line 0.
  unsigned LastAsmLine =
    Asm->OutStreamer->getContext().getCurrentDwarfLoc().getLine();

Differential revision: https://reviews.llvm.org/D56767

llvm-svn: 351395