[llvm][SVE] Addressing mode for FF/NF loads.

Summary:
This patch adds addressing mode computation for the following SVE
instructions:

* ldff1{s}<T1> { <Zt>.<T2> }, <Pg>/Z, [<Xn|SP>{, <Xm>{, lsl #imm}}]
* ldnf1{s}<T1> { <Zt>.<T2> }, <Pg>/Z, [<Xn|SP>{, #<imm>, mul vl}]

Reviewers: andwar, sdesmalen, rengolin, efriedma

Subscribers: tschuett, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76209