[SROA] Clean up some really weird code, no functionality changed.

We already have the instruction extracted into 'I', just cast that to
a store the way we do for loads. Also, we don't enter the if unless SI
is non-null, so don't test it again for null.

I'm pretty sure the entire test there can be nuked, but this is just the
trivial cleanup.

llvm-svn: 263112