[AArch64] Avoid partial register deps on insertelt of load into lane 0.

This improves upon r246462: that prevented FMOVs from being emitted
for the cross-class INSERT_SUBREGs by disabling the formation of
INSERT_SUBREGs of LOAD.  But the ld1.s that we started selecting
caused us to introduce partial dependencies on the vector register.

Avoid that by using SCALAR_TO_VECTOR: it's a first-class citizen that
is folded away by many patterns, including the scalar LDRS that we
want in this case.

Credit goes to Adam for finding the issue!

llvm-svn: 299482