[InstCombine] limit extracting shuffle transform based on uses

[InstCombine] limit extracting shuffle transform based on uses

As discussed in D53037, this can lead to worse codegen, and we
don't generally expect the backend to be able to optimize
arbitrary shuffles. If there's only one use of the 1st shuffle,
that means it's getting removed, so that should always be
safe.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353235 91177308-0d34-0410-b5e6-96231b3b80d8