[AArch64] Fix FMLS scalar-indexed-from-2s-after-neg patterns.

We canonicalize V64 vectors to V128 through insert_subvector: the other
FMLA/FMLS/FMUL/FMULX patterns match that already, but this one doesn't,
so we'd fail to match fmls and generate fneg+fmla instead.
The vector equivalents are already tested and functional.

llvm-svn: 245107