Currently iterators over EquivalenceClasses will iterate over std::set,
which guarantees the order specified by the comperator. Unfortunately in
many cases, EquivalenceClasses are used with pointers, so iterating over
std::set of pointers will not be deterministic across runs.

There are multiple places that explicitly try to sort the equivalence
classes before using them to try to get a deterministic order
(LowerTypeTests, SplitModule), but there are others that do not at the
moment and this can result at least in non-determinstic value naming in
Float2Int.

This patch updates EquivalenceClasses to keep track of all members via a
extra SmallVector and removes code from LowerTypeTests and SplitModule
to sort the classes before processing.

Overall it looks like compile-time slightly decreases in most cases, but
close to noise:

https://llvm-compile-time-tracker.com/compare.php?from=7d441d9892295a6eb8aaf481e1715f039f6f224f&to=b0c2ac67a88d3ef86987e2f82115ea0170675a17&stat=instructions

PR: https://github.com/llvm/llvm-project/pull/134075

Remove a level of indirection and update code to use range-based for
loops.

Removes a redundant lookup in the mapping.:

Identified with misc-include-cleaner.

The new round-robin algorithm overrides the hash-based distribution of
functions to modules. It achieves a more even number of functions per
module when the number of functions is close to the number of requested
modules. It's not in use by default and is available under a new flag.

Fix repeated map lookup, variable name, formatting and a missing space.

Identified with readability-qualified-auto.

As discussed in:
* https://reviews.llvm.org/D94166
* https://lists.llvm.org/pipermail/llvm-dev/2020-September/145031.html

The GlobalIndirectSymbol class lost most of its meaning in
https://reviews.llvm.org/D109792, which disambiguated getBaseObject
(now getAliaseeObject) between GlobalIFunc and everything else.
In addition, as long as GlobalIFunc is not a GlobalObject and
getAliaseeObject returns GlobalObjects, a GlobalAlias whose aliasee
is a GlobalIFunc cannot currently be modeled properly. Creating
aliases for GlobalIFuncs does happen in the wild (e.g. glibc). In addition,
calling getAliaseeObject on a GlobalIFunc will currently return nullptr,
which is undesirable because it should return the object itself for
non-aliases.

This patch refactors the GlobalIFunc class to inherit directly from
GlobalObject, and removes GlobalIndirectSymbol (while inlining the
relevant parts into GlobalAlias and GlobalIFunc). This allows for
calling getAliaseeObject() on a GlobalIFunc to return the GlobalIFunc
itself, making getAliaseeObject() more consistent and enabling
alias-to-ifunc to be properly modeled in the IR.

I exercised some judgement in the API clients of GlobalIndirectSymbol:
some were 'monomorphized' for GlobalAlias and GlobalIFunc, and
some remained shared (with the type adapted to become GlobalValue).

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D108872

To better reflect the meaning of the now-disambiguated {GlobalValue,
GlobalAlias}::getBaseObject after breaking off GlobalIFunc::getResolverFunction
(D109792), the function is renamed to getAliaseeObject.

While both GlobalAlias and GlobalIFunc are GlobalIndirectSymbol, their
`getIndirectSymbol()` usage is quite different (GlobalIFunc's resolver
is an entity different from GlobalIFunc itself).

As discussed on https://lists.llvm.org/pipermail/llvm-dev/2020-September/144904.html
("[IR] Modelling of GlobalIFunc"), the name `getBaseObject` is confusing when
used with GlobalIFunc.

To resolve the confusion:

* Move GloalIndirectSymol::getBaseObject to GlobalAlias:: (GlobalIFunc should use `getResolver` instead)
* Change GlobalValue::getBaseObject not to inspect GlobalIFunc. Note: the function has 7 references.
* Add GlobalIFunc::getResolverFunction to peel off potential ConstantExpr indirection
  (`strlen` in `test/LTO/Resolution/X86/ifunc.ll`)

Note: GlobalIFunc::getResolver (like GlobalAlias::getAliasee which does not peel
off ConstantExpr indirection) is kept to be used by ValueEnumerator.

Reviewed By: ibookstein

Differential Revision: https://reviews.llvm.org/D109792