Use intrinsic expressions for most intrinsics (#4000)

* SizeOf: Use intrinsic expression instead of IntrinsicModule.
* mux2cell, mux4cell: Use intrinsic expression.
* isX: use intrinsic expression.
* PlusArgsValue: use intrinsic expression
* PlusArgsTest: IntrinsicExpr, drop unused type argument.  Preserve old interface but mark deprecated.
* clock gate: use intrinsic expression.
* tests: split out FIRRTL generation test from compile check.