[MachineOutliner][NFC] Don't map MBBs that don't contain legal instructions

I noticed that there are lots of basic blocks that don't have enough legal
instructions in them to warrant outlining. We can skip mapping these entirely.

In sqlite3, compiled for AArch64 at -Oz, this results in a 10% reduction of
the total nodes in the suffix tree. These nodes can never be part of a
repeated substring, and so they don't impact the result at all.

Before this, there were 62128 nodes in the tree for sqlite3. After this, there
are 56457 nodes.

llvm-svn: 346373