[NFC][AArch64] Fix vector vsqadd intrinsics operands

Summary:
Change the vsqadd vector instrinsics to have the second argument as signed values, not unsigned,
accordingly to https://developer.arm.com/architectures/instruction-sets/simd-isas/neon/intrinsics

Reviewers: LukeCheeseman, ostannard

Reviewed By: ostannard

Subscribers: javed.absar, kristof.beyls, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64210

llvm-svn: 365608