[ARM64] Fix materialization of an fp128 zero immediate. There currently
is not a pattern to lower this with clever instructions that zero the
register, so restrict the zero immediate legality special case to f64
and f32 (the only two sizes which fmov seems to directly support). Fixes
backend errors when building code such as libxml.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205161 91177308-0d34-0410-b5e6-96231b3b80d8