A new hidden option exec-on-ir-change=exe that calls exe each time IR changes

Summary:
A new option exec-on-ir-changed is defined that allows one to specify an
exe that is called after each pass in the opt pipeline that changes the IR.
The exec-on-ir-change=exe option saves the IR in a temporary file and calls exe
with the name of the file and the name of the pass that just changed it after
each pass alters the IR. exe is also called with the initial IR. This
can be used, for example, to determine which pass corrupts the IR by having
exe as a script that calls llc and runs a test to see after which pass the
results change. The print-changed filtering options are respected.  Note that
this is only supported with the new pass manager.

Author: Jamie Schmeiser <schmeise@ca.ibm.com>
Reviewed By: aeubanks (Arthur Eubanks)
Differential Revision: https://reviews.llvm.org/D110776