[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