llvm dev - Jan 2021 - [GSoC'20]Improve MegreFunctions to incorporate MergeSimilarFunctions patches and ThinLTO Support

Hello,

Briefly summarizing the results of my GSoC'20 project here as it'll be
easy
for someone working in the future to search.

* A protected method has been declared in each subclass of the IR which
says how two instructions of a specific IR type can be compared. This will
help in introducing total ordering between IR instructions during
comparison to merge. Also, a method has been added in the
"Instruction"
class which compares two IR instructions making use of the newly added
protected method. This approach minimizes the number of files to touch for
a change in IR instructions. But, when there's a new operand is added to an
instruction and the comparator method has not been updated, there's no
error generated but this can be easily checked as the comparator method
lies in the subclass level which is where the new operand will have to be
added. Testcases have been added for each IR type to avoid such a scenario.
This approach provides methods which act as a centralized place for IR
comparison.

* The MergeSimilarFunctions pass has been integrated into the
MergeFunctions pass. The pass now needs a parameter which tells whether to
do only precise merging(merge exact replicates of functions) or partial
merging(merge functions which are similar) too. The MergeSimilarFunctions
makes use of a newly added class, PartialFunctionComparator, which compares
functions and stores the differing instructions and also records the degree
of similarity between functions. Both passes delegate the comparison of IR
to the newly added protected methods(through FunctionComparator classes)
for IR comparison so that there won't be any mis-compiles if the comparator
method is updated.

*Patches*
*1) *https://reviews.llvm.org/D82892 ([NFC] Methods to compare IR added in
each IR subclass)
2) https://reviews.llvm.org/D85616 (Improved MergeFunctions to merge
similar functions)

The benchmarks and a much more detailed report explaining the alternate
approach tried can be found at
https://gist.github.com/vish99/7f1454a03d78d466989b6c982ccf59dc
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