[AArch64][SVE] NFC: Rename isOrig -> isReverseInstr

This is a non-functional to clarify some of the terminology in the
AArch64SVEInstrInfo/SVEInstrFormats.td files around the tables
for mapping an instruction to it's reverse instruction counter part,
and vice versa. e.g. DIV -> DIVR and DIVR -> DIV.

Reviewers: paulwalker-arm, cameron.mcinally, rengolin, efriedma

Reviewed By: paulwalker-arm, efriedma

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82979