add TEnumIndex and make matcher generate that instead of Type.enumIndex call

[nadako]

This adds TEnumIndex of texpr node and makes pattern-matcher generate it instead of Type.enumIndex call, as well as adds handling of that node to target generators. Adding this introduced some optimizations that eliminated a couple switches on some targets, also surfacing #6352 on C# (disabled the test for now).

I didn't remove older enum index hacks and Type.enumIndex for now, and there's no way to generate TEnumIndex from haxe code right now, so Type.enumIndex implementations are duplicated. I tried to add matching on EnumValueTools.getIndex() and using that for Type.enumIndex but that loses the enum expression type making it EnumValue instead, so this is something we have to think about.

Also, I have to check out the diffs again, but creating this PR nevertheless so Travis can show if everything is okay.

[Simn]

It's missing here:

haxe/src/optimization/analyzer.ml

Line 447 in c3dc791

let e1 = match ctx.com.platform,e.eexpr with

But I'm fine with merging this as soon as the CI is green so we can take it from there.

[nadako]

No it's not :)

haxe/src/optimization/analyzer.ml

Line 423 in c3dc791

| TEnumIndex e1 ->

[nadako]

Huh, Lua is failing with some weird error Type not found : Issue6276

[Simn]

Ah I see. Looking forward to cleaning up that mess.

I wonder why it checks the type for TEnum in the current implementation.