[AArch64] Implement the isTruncateFree API.

In AArch64 i64 to i32 truncate operation is a subregister access.

This allows more opportunities for LSR optmization to eliminate
variables of different types (i32 and i64).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205925 91177308-0d34-0410-b5e6-96231b3b80d8