[AArch64] Fix vsqadd scalar intrinsics operands

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

The existing unsigned argument can cause faulty code as negative float to unsigned conversion is
undefined, which llvm/clang optimizes away.

Reviewers: LukeCheeseman, john.brawn

Reviewed By: john.brawn

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64239

llvm-svn: 365298