[analyzer] Fix crash inside RangeConstraintManager.cpp introduced by D112621

[analyzer] Fix crash inside RangeConstraintManager.cpp introduced by D112621

It seems like `LHS` and `RHS` could be empty range sets.
This caused an assertion failure inside RangeConstraintManager.

I'm hoisting out the check from the function into the call-site.
This way we could assert that we only want to deal with non-empty range
sets.

The relevant part of the trace:
```
 #6 0x00007fe6ff5f81a6 __assert_fail_base (/lib64/libc.so.6+0x2f1a6)
 #7 0x00007fe6ff5f8252 (/lib64/libc.so.6+0x2f252)
 #8 0x00000000049caed2 (anonymous namespace)::SymbolicRangeInferrer::VisitBinaryOperator(clang::ento::RangeSet, clang::BinaryOperatorKind, clang::ento::RangeSet, clang::QualType) RangeConstraintManager.cpp:0:0
 #9 0x00000000049c9867 (anonymous namespace)::SymbolicRangeInferrer::infer(clang::ento::SymExpr const*) RangeConstraintManager.cpp:0:0
#10 0x00000000049bebf5 (anonymous namespace)::RangeConstraintManager::assumeSymNE(llvm::IntrusiveRefCntPtr<clang::ento::ProgramState const>, clang::ento::SymExpr const*, llvm::APSInt const&, llvm::APSInt const&) RangeConstraintManager.cpp:0:0
#11 0x00000000049d368c clang::ento::RangedConstraintManager::assumeSymUnsupported(llvm::IntrusiveRefCntPtr<clang::ento::ProgramState const>, clang::ento::SymExpr const*, bool) (../../main-github/llvm/build-all/bin/clang+0x49d368c)
#12 0x00000000049f0b09 clang::ento::SimpleConstraintManager::assumeAux(llvm::IntrusiveRefCntPtr<clang::ento::ProgramState const>, clang::ento::NonLoc, bool) (../../main-github/llvm/build-all/bin/clang+0x49f0b09)
#13 0x00000000049f096a clang::ento::SimpleConstraintManager::assume(llvm::IntrusiveRefCntPtr<clang::ento::ProgramState const>, clang::ento::NonLoc, bool) (../../main-github/llvm/build-all/bin/clang+0x49f096a)
#14 0x00000000049f086d clang::ento::SimpleConstraintManager::assumeInternal(llvm::IntrusiveRefCntPtr<clang::ento::ProgramState const>, clang::ento::DefinedSVal, bool) (../../main-github/llvm/build-all/bin/clang+0x49f086d)
#15 0x000000000492d3e3 clang::ento::ConstraintManager::assumeDual(llvm::IntrusiveRefCntPtr<clang::ento::ProgramState const>, clang::ento::DefinedSVal) (../../main-github/llvm/build-all/bin/clang+0x492d3e3)
#16 0x0000000004955b6d clang::ento::ExprEngine::evalEagerlyAssumeBinOpBifurcation(clang::ento::ExplodedNodeSet&, clang::ento::ExplodedNodeSet&, clang::Expr const*) (../../main-github/llvm/build-all/bin/clang+0x4955b6d)
#17 0x00000000049514b6 clang::ento::ExprEngine::Visit(clang::Stmt const*, clang::ento::ExplodedNode*, clang::ento::ExplodedNodeSet&) (../../main-github/llvm/build-all/bin/clang+0x49514b6)
#18 0x000000000494c73e clang::ento::ExprEngine::ProcessStmt(clang::Stmt const*, clang::ento::ExplodedNode*) (../../main-github/llvm/build-all/bin/clang+0x494c73e)
#19 0x000000000494c459 clang::ento::ExprEngine::processCFGElement(clang::CFGElement, clang::ento::ExplodedNode*, unsigned int, clang::ento::NodeBuilderContext*) (../../main-github/llvm/build-all/bin/clang+0x494c459)
#20 0x000000000492f3d0 clang::ento::CoreEngine::HandlePostStmt(clang::CFGBlock const*, unsigned int, clang::ento::ExplodedNode*) (../../main-github/llvm/build-all/bin/clang+0x492f3d0)
#21 0x000000000492e1f6 clang::ento::CoreEngine::ExecuteWorkList(clang::LocationContext const*, unsigned int, llvm::IntrusiveRefCntPtr<clang::ento::ProgramState const>) (../../main-github/llvm/build-all/bin/clang+0x492e1f6)
```

Differential Revision: https://reviews.llvm.org/D112621