[MachineOutliner][NFC] Don't map more illegal instrs than you have to

We were mapping an instruction every time we saw something we couldn't map
before this. Since each illegal mapping is unique, we only have to do this once.

This makes it so that we don't map illegal instructions when the previous
mapped instruction was illegal.

In CTMark (AArch64), this results in 240 fewer instruction mappings on
average over 619 files in total. The largest improvement is 12576 fewer
mappings in one file, and the smallest is 0. The median improvement is 101
fewer mappings.

llvm-svn: 342405