[ARM][MVE] Enable truncating masked stores

Allow us to generate truncating masked store which take v4i32 and
v8i16 vectors and can store to v4i8, v4i16 and v8i8 and memory.
Removed support for unaligned masked stores.

Differential Revision: https://reviews.llvm.org/D68461

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375108 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed