[FastISel][AArch64] Use the proper FMOV instruction to materialize a +0.0.

Use FMOVWSr/FMOVXDr instead of FMOVSr/FMOVDr, which have the proper register
class to be used with the zero register. This makes the MachineInstruction
verifier happy again.

This is related to <rdar://problem/18027157>.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216040 91177308-0d34-0410-b5e6-96231b3b80d8