[HotColdSplit] Split more aggressively before/after cold invokes

While a cold invoke itself and its unwind destination can't be
extracted, code which unconditionally executes before/after the invoke
may still be profitable to extract.

With cost model changes from D57125 applied, this gives a 3.5% increase
in split text across LNT+externals on arm64 at -Os.

llvm-svn: 352160