Use intrinsic pattern to make a simpler match

Use intrinsic pattern to make a simpler match

This is a follow-on to r236740 where I took Andrea's advice
in D9504 to remove a redundant pattern...except that I removed
the wrong pattern!

AFAICT, there is no change in the final code produced because
subsequent passes would clean up the extra instructions created
by the more complicated pattern.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236743 91177308-0d34-0410-b5e6-96231b3b80d8