* Early evaluation of mIs
The `evalIs` implementation was just a broken copy of `isOpImpl` so
let's just avoid it alltogether: `mIs` nodes are either resolved during
the semantic phase or bust.
* Remove dead code and tidy it up
* Don't consider tyAnd/tyNot/tyOr/tyAnything as generic
`containsGenericType` was too shallow and didn't check all the branches.
The resulting half-processed nodes are often simplified by the constant
folding pass but when that's not possible we get a nasty error during
codegen.
Fixes#8693
* Move the blame onto the semFold pass
Slightly better evaluation of `is` forms.
