[AArch64] Refactor floating point materialization. NFC

It splits the login of actual instruction emission away from the logic
that figures out the appropriate sequence on AArch64ExpandPseudo::expandMOVImm.
The new function AArch64_IMM::expandMOVImm, which return the list of the 
instructions to materialize the immediate constant, is implemented on a 
separated unit because it will be used in a subsequent patch to optimize
floating point materialization.

Reviewers: efriedma

Differential Revision: https://reviews.llvm.org/D58915

llvm-svn: 356387