MFC r333715:

MFC r333715:

Pull in r322325 from upstream llvm trunk (by Matthias Braun):

  PeepholeOpt cleanup/refactor; NFC

  - Less unnecessary use of `auto`
  - Add early `using RegSubRegPair(AndIdx) =` to avoid countless
    `TargetInstrInfo::` qualifications.
  - Use references instead of pointers where possible.
  - Remove unused parameters.
  - Rewrite the CopyRewriter class hierarchy:
     - Pull out uncoalescable copy rewriting functionality into
       PeepholeOptimizer class.
     - Use an abstract base class to make it clear that rewriters are
       independent.
  - Remove unnecessary \brief in doxygen comments.
  - Remove unused constructor and method from ValueTracker.
  - Replace UseAdvancedTracking of ValueTracker with DisableAdvCopyOpt
    use.

Even though upstream marked this as "No Functional Change", it does
contain some functional changes, and these fix a compiler hang for one
particular source file in the devel/godot port.

Approved by:	re (kib)
PR:		228261