[AArch64] Fix offset calculation

r374772 changed Offset to be an int64_t but left NewOffset as an int.
Scale is unsigned, so in the calculation `Offset - NewOffset * Scale`,
`NewOffset * Scale` was promoted to unsigned and was then zero-extended
to 64 bits, leading to an incorrect computation which manifested as an
out-of-memory when building the Swift standard library for Android
aarch64. Promote NewOffset to int64_t to fix this, and promote
EmittableOffset as well, since its one user passes it to a function
which takes an int64_t anyway.

Test case based on a suggestion by Sander de Smalen!

Differential Revision: https://reviews.llvm.org/D69018

llvm-svn: 375043