[llvm][CodeGen] Rename SVE gather prefetch intrinsics. [NFC]

[llvm][CodeGen] Rename SVE gather prefetch intrinsics. [NFC]

Summary:
The renaming is necessary to make the naming scheme uniform with other
gather/scatter load/stores SVE intrinsics.

The naming of variables and functions have been adapted to make it
explicit whether we are dealing with a scalar offset (which is
unscaled) or an index (which is scaled according to the data type of
the lanes of the vector).

Reviewers: andwar, sdesmalen, rengolin

Reviewed By: andwar

Subscribers: tschuett, hiraditya, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77839