[ARM] Add data gathering hint instruction

Summary:
This patch upstreams support the optional ARMv8.0 Data Gathering Hint (DGH)
extension, which adds the Data Gathering Hint instruction to the hint
space.

See ARMv8.0-DGH in the Arm Architecture Reference Manual Armv8 for more
information.

Reviewers: t.p.northover, rengolin, SjoerdMeijer, ab, danielkiss, samparker

Reviewed By: SjoerdMeijer

Subscribers: LukeGeeson, ostannard, kristof.beyls, hiraditya, danielkiss, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77097