[AArch64] Rename Pairs to Mappings in AArch64NamedImmMapper

Third element is to be added soon to "struct AArch64NamedImmMapper::Mapping". So its instances are renamed from ...Pairs to ...Mappings

Reviewers: jmolloy

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8582

llvm-svn: 233300