Remove sometimes faulty rewrite of memcpy in instcombine.

Summary:
Solves PR 31990.

The bad rewrite could replace a memcpy of one word with
 store i4 -1
while it should actually be
 store i8 -1

Hopefully opt and llc has improved enough so the original optimization
done by the code isn't needed anymore.

One already existing testcase is affected. It originally tested that
the memcpy was replaced with
 load double
but since we now remove that rewrite it will be
 load i64
instead.

Patch suggestion by Eli Friedman.

Reviewers: eli.friedman, majnemer, efriedma

Reviewed By: efriedma

Subscribers: efriedma, llvm-commits

Differential Revision: https://reviews.llvm.org/D30254

llvm-svn: 296585