The link should refer to the section of 'phi' Instruction in the
LangRef, but it referred to the subsection of 'fcmp' Instruction.
Replace it with appropriate one.

Currently the pass documentation specifies every pass with a single dash
(-) in front of it as this was the old format for specifying passes with
opt. However, after the transition to the new PM, this syntax is
deprecated. This patch removes all the single dashes from the pass page
to reflect this. Additionally, some previously removed/renamed passes
are removed/updated in the pass list.

Differential Revision: https://reviews.llvm.org/D157656

Includes 2 corrections:

 * Update irreducible control flow and add references to CycleTerminology;
   Natural loop is not the only definition of something looping in LLVM anymore.

 * Mention mustprogress loop and function attributes to be used
   instead of the llvm.sideeffect intrinsic.

`opt -analyze` is legacy PM-specific. Show better ways of doing the same
thing, generally with some sort of `-passes=print<foo>`.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D119486

Differential Revision: https://reviews.llvm.org/D89739

Motivated by D88183, this seeks to clarify the current loop nomenclature with added illustrations, examples for possibly unexpected situations (infinite loops not part of the "parent" loop, logical loops sharing the same header, ...), and clarification on what other sources may consider a loop. The current document also has multiple errors that are fixed here.

Some selected errors:
 * Loops a defined as strongly-connected components. A component a partition of all nodes, i.e. a subloop can never be a component. That is, the document as it currently is only covers top-level loops, even it also uses the term SCC for subloops.
 * "a block can be the header of two separate loops at the same time" (it is considered a single loop by LoopInfo)
 * "execute before some interesting event happens" (some interesting event is not well-defined)

Reviewed By: baziotis, Whitney

Differential Revision: https://reviews.llvm.org/D88408