[ubsan] Assert that each check only has one SanitizerKind

The `Checked` parameter of `CodeGenFunction::EmitCheck` is of type `ArrayRef<std::pair<llvm::Value *, SanitizerMask>>`. In the general case, SanitizerMask can denote that zero or more sanitizers are enabled, but I believe (from tests and inspecting the code) that `EmitCheck` assumes exactly one sanitizer enabled per SanitizerMask. This patch adds an assertion for this invariant.

This is not intended to change the functionality of the code, but will
make it easier for maintainers to reason about and extend the
`EmitCheck` function.

Author: thurstond

clang/lib/CodeGen/CGExpr.cpp

@@ -3603,6 +3603,8 @@ void CodeGenFunction::EmitCheck(
   llvm::Value *TrapCond = nullptr;
   bool NoMerge = false;
   for (int i = 0, n = Checked.size(); i < n; ++i) {
+    assert(Checked[i].second.isPowerOf2());
+
     llvm::Value *Check = Checked[i].first;
     // -fsanitize-trap= overrides -fsanitize-recover=.
     llvm::Value *&Cond =