[analyzer] alpha.core.Conversion - Fix false positive for 'U32 += S16;'...

[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

llvm-svn: 299523