[analyzer] alpha.core.Conversion - Fix false positive for 'U32 += S16;' expression, that is not unsafe

Summary:
The alpha.core.Conversion was too strict about compound assignments and could warn even though there is no problem.

Differential Revision: https://reviews.llvm.org/D25596



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299523 91177308-0d34-0410-b5e6-96231b3b80d8