AArch64: fix build on some MSVC versions

This does two things:

It removes a call to abs() which may have "long long" parameter on Windows,
which is not necessarily available in C++03.

It also corrects the signedness of Amount, which was relying on
implementation-defined conversions previously.

Code was already tested (albeit in an implemnetation defined way) so no extra
tests.

llvm-svn: 174885