[CodeGen] Split out cold exception handling pads.
Support for splitting exception handling pads was added in D73739. This
change updates the code to split out exception handling pads if profile
information indicates that they are cold. For a given function with
multiple landind pads, if one of them is hot they are all retained as
part of the hot code section.
Differential Revision: https://reviews.llvm.org/D96372