How to find SourceRange for commas between StmtConditionElements?

I'm working on a migration pass. Given code like this:

if let x = something,
   let y = somethingElse
{ }

I want to remove the let y line entirely, leaving me with just this:

if let x = something
{ }

Each item within the IfStmt is a StmtConditionElement, which has its own SourceRange. That's working great. I can identify the source range of the let y = somethingElse range. But is there a way to find my way back to the preceding comma, so that I can leave things in a constant state? I'm not finding anything on either IfStmt or StmtConditionElement that would indicate the location of the commas.