Make the FunctionComparator of the MergeFunctions pass a stand-alone utility.

This is pure refactoring. NFC.

This change moves the FunctionComparator (together with the GlobalNumberState
utility) in to a separate file so that it can be used by other passes.
For example, the SwiftMergeFunctions pass in the Swift compiler:
https://github.com/apple/swift/blob/master/lib/LLVMPasses/LLVMMergeFunctions.cpp

Details of the change:

*) The big part is just moving code out of MergeFunctions.cpp into FunctionComparator.h/cpp
*) Make FunctionComparator member functions protected (instead of private)
   so that a derived comparator class can use them.

Following refactoring helps to share code between the base FunctionComparator
class and a derived class:

*) Add a beginCompare() function
*) Move some basic function property comparisons into a separate function compareSignature()
*) Do the GEP comparison inside cmpOperations() which now has a new
   needToCmpOperands reference parameter

https://reviews.llvm.org/D25385

llvm-svn: 286632