[MachineOutliner][NFC] Only map blocks which have adjacent legal instructions

If a block doesn't have any ranges of adjacent legal instructions, then it
can't have outlining candidates. There's no point in mapping legal isntructions
in situations like this.

I noticed this reduces the size of the suffix tree in sqlite3 for AArch64 at
-Oz by about 3%.

llvm-svn: 346379