[llvm][CodeGen] Rename SVE gather prefetch intrinsics. [NFC]
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
Differential Revision: https://reviews.llvm.org/D77839