[HotColdSplitting] Refine definition of unlikelyExecuted

The splitting pass uses its 'unlikelyExecuted' predicate to statically
decide which blocks are cold.

- Do not treat noreturn calls as if they are cold unless they are actually
  marked cold. This is motivated by functions like exit() and longjmp(), which
  are not beneficial to outline.

- Do not treat inline asm as an outlining barrier. In practice asm("") is
  frequently used to inhibit basic block merging; enabling outlining in this case
  results in substantial memory savings.

- Treat invokes of cold functions as cold.

As a drive-by, remove the 'exceptionHandlingFunctions' predicate, because it's
no longer needed. The pass can identify & outline blocks dominated by EH pads,
so there's no need to special-case __cxa_begin_catch etc.

Differential Revision: https://reviews.llvm.org/D54244

llvm-svn: 348640