[AArch64] Fix scalar vuqadd intrinsics operands

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

So now the compiler correctly warns that a undefined negative float conversion is being done.

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/D64242

llvm-svn: 365300