Comma expressions need to special-handle function calls. When validating
the rhs of a Comma, the validatior needs to ensure that it returns
AsmType::Float() if the function being called is fround().

BUG= https://bugs.chromium.org/p/v8/issues/detail?id=5528

Review-Url: https://chromiumcodereview.appspot.com/2426473007
Cr-Commit-Position: refs/heads/master@{#40408}