Fix SROA to avoid unnecessary scalar conversions for 1-element vectors.

When a 1-element vector alloca is promoted, a store instruction can often be
rewritten without converting the value to a scalar and using an insertelement
instruction to stuff it into the new alloca.  This patch just adds a check
to skip that conversion when it is unnecessary.  This turns out to be really
important for some ARM Neon operations where <1 x i64> is used to get around
the fact that i64 is not a legal type.