[SEMA] add more -Wfloat-conversion to compound assigment analysis

Summary: Fixes Bug: https://bugs.llvm.org/show_bug.cgi?id=27061

Reviewers: aaron.ballman, acoomans

Reviewed By: aaron.ballman, acoomans

Subscribers: acoomans, cfe-commits, srhines, pirama

Differential Revision: https://reviews.llvm.org/D50467

llvm-svn: 339581