Faster ConstructorInfo.Invoke (#86855)

Faster ConstructorInfo.Invoke (#86855)

* Faster ConstructorInfo.Invoke

Speeds up reflection-invoking constructors by about 30%.

Noticed we spend some time in the `ConstructorInfo` invocation infrastructure for no good reason.

* We'd first allocate an instance of the object using a general purpose allocator (that first needs to dissect the `MethodTable` to figure out how to allocate), and then
* We'd call into general-purpose Invoke infrastructure that would validate the `this` is valid.

Neither of those are necessary - we can figure out the right allocator at the time the method invoker is first accessed, and validating `this` is not necessary because we _just_ allocated the right one.

* This is reachable and the old behavior was needed

* Apply suggestions from code review

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Co-authored-by: Jan Kotas <jkotas@microsoft.com>