[Sema] fix -Wfloat-conversion test case.

Summary:
Fixes r339581 ("[SEMA] add more -Wfloat-conversion to
compound assigment analysis").

This test case was caught in postsubmit testing.

Reviewers: aaron.ballman, gkistanova

Reviewed By: aaron.ballman

Subscribers: cfe-commits, srhines

Differential Revision: https://reviews.llvm.org/D50647

llvm-svn: 339593