Instead of printing unnecessary basic block labels as labels in

Instead of printing unnecessary basic block labels as labels in
verbose-asm mode, print comments instead. This eliminates a non-comment
difference between verbose-asm mode and non-verbose-asm mode.

Also, factor out the relevant code out of all the targets and into
target-independent code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@83392 91177308-0d34-0410-b5e6-96231b3b80d8