[globalisel][aarch64] Replace magic numbers with corresponding enumerators in ValMappings. NFC

Reviewers: ab, t.p.northover, qcolombet

Subscribers: aemerson, rengolin, vkalintiris, dberris, llvm-commits, rovka

Differential Revision: https://reviews.llvm.org/D27336

llvm-svn: 288810