[AArch64][SVE2] Minor refactoring and cleanup

Summary:
* Clarify comment with SVE2 for predicated shifts and move next to other
  shift instructions.
* Clarify comments for various instructions.
* Move FCVTX instruction next to other fp conversions.
* Move FLOGB to next to other fp instructions and fix description.
* Remove "cons" from non-constructive multiclass for bitwise shift-right
  and accumulate instructions.

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D65390

llvm-svn: 367396