Improve isProducedBySanitizer to detect more cases of values produced by a call to a sanitizer

[mlevesquedion]

Consider this function:

func TestSanitizedSourceDoesNotTriggerFindingWhenTypeAsserted(s *core.Source) {
	sanitized := core.Sanitize(s)[0].(*core.Source)
	core.Sinkf("Sanitized %v", sanitized)
}

The relevant part of the SSA graph is:

The issue here is that the TypeAssert is not identified as being produced by a sanitizer according to isProducedBySanitizer. We still don't produce a report here because the current propagation code finds the sanitizer and determines that the sinked value is dominated by the sanitizing call.

This incorrect behavior may lead to false positives.