[AArch64] Move initializations of AArch64NamedImmMapper out of void AArch64Operand::print(...)

class AArch64NamedImmMapper is to become dependent of SubTargetFeatures, while class AArch64Operand don't have access to the latter. 

So, AArch64NamedImmMapper constructor invocations are refactored away from methods of AArch64Operand.

Reviewers: jmolloy

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8579

llvm-svn: 233297