Introduce a Loop::getLocStr stolen from LoopVectorize's static function
getDebugLocString in order to have uniform debug output headers across
LoopVectorize, LoopAccessAnalysis, and LoopDistribute. The motivation
for this change is to have UpdateTestChecks recognize the headers and
automatically generate CHECK lines for debug output, with minimal
special-casing.

- There is no restriction on a loop with controlled convergent
operations when
  the relevant tokens are defined and used within the loop.

- When a token defined outside a loop is used inside (also called a loop
convergence heart), unrolling is allowed only in the absence of
remainder or
  runtime checks.

- When a token defined inside a loop is used outside, such a loop is
said to be
"extended". This loop can only be unrolled by also duplicating the
extended part
  lying outside the loop. Such unrolling is disabled for now.

- Clean up loop hearts: When unrolling a loop with a heart, duplicating
the
heart will introduce multiple static uses of a convergence control token
in a
cycle that does not contain its definition. This violates the static
rules for
tokens, and needs to be cleaned up into a single occurrence of the
intrinsic.

- Spell out the initializer for UnrollLoopOptions to improve
readability.


Original implementation [D85605] by Nicolai Haehnle
<nicolai.haehnle@amd.com>.

I'm planning to remove StringRef::equals in favor of
StringRef::operator==.

- StringRef::operator==/!= outnumber StringRef::equals by a factor of
  53 under llvm/ in terms of their usage.

- The elimination of StringRef::equals brings StringRef closer to
  std::string_view, which has operator== but not equals.

- S == "foo" is more readable than S.equals("foo"), especially for
  !Long.Expression.equals("str") vs Long.Expression != "str".

The legacy pass manager printed the function name when printing
loop info (via -analyze option). Like this:

  Printing analysis 'Natural Loop Information' for function 'func':
  Loop at depth 1 containing: ...
      Loop at depth 2 containing: ...

Make sure we print such a first line including the function name
also when using the new pass manager version of LoopPrinterPass.

The format of the string is changed slightly, so now we say:

  Loop info for function 'func':
  Loop at depth 1 containing: ...
      Loop at depth 2 containing: ...

This was originally requested in
  https://discourse.llvm.org/t/need-usage-help-w-new-pass-manager-for-opt-analysis-natural-loop-information/75874/7
and also mentioned in
  https://github.com/llvm/llvm-project/issues/76762

This patch replaces uses of StringRef::{starts,ends}with with
StringRef::{starts,ends}_with for consistency with
std::{string,string_view}::{starts,ends}_with in C++20.

I'm planning to deprecate and eventually remove
StringRef::{starts,ends}with.

Loop iteration utilities do not change LoopInfo.

This commit splits the generic part of `LoopInfo` into separate files.
These new `GenericLoopInfo` files are located in `llvm/Support` to be inline
with `GenericDomTree`.

Furthermore, this change ensures that MLIR's Bazel build does not have
to link against `LLVMAnalysis` just to use these template headers.

Depends on D148219

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D148235