[Analyzer] [HOTFIX!] SValBuilder crash when `aggressive-binary-operation-simplification` enabled

During the review of D41938 a condition check with an early exit accidentally
slipped into a branch, leaving the other branch unprotected. This may result in
an assertion later on. This hotfix moves this contition check outside of the
branch.

Differential Revision: https://reviews.llvm.org/D55051

llvm-svn: 348362