[ARM64] Fix immediate cost calculation for types larger than i64.

The immediate cost calculation code was hitting an assertion in the included
test case, because APInt was still internally 128-bits. Truncating it to 64-bits
fixed the issue.

Fixes <rdar://problem/16572521>.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205947 91177308-0d34-0410-b5e6-96231b3b80d8