[SVE] Add patterns for MUL immediate instruction.

Summary: Add the missing MUL pattern for integer immediate instructions.

Reviewers: sdesmalen, huntergr, efriedma, c-rhodes, kmclaughlin

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72654