AArch64: fix conversion of 'J' inline asm constraints.

'J' represents a negative number suitable for an add/sub alias
instruction, but while preparing it to become an int64_t we were
mangling the sign extension. So "i32 -1" became 0xffffffffLL, for
example.

Should fix one half of PR20456.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214052 91177308-0d34-0410-b5e6-96231b3b80d8