[Parser] Simplify sequence expression parsing

`SequenceExprSyntax` now gurantees that it has odd number elements using
`UnresolvedTernaryExpr`/`UnresolvedAsExpr`/`UnresolvedIsExpr`.

Factored out the operator parsing into `parseSequenceExpressionOperator()`
which returns the operator expresssion, and optionally the RHS, so we can
avoid nested loops.
Also, `parseSequenceExprssion()` now has a performance optimization that
avoids heap `Array` construction unless the expression is actually
sequenced.

Author: rintaro