Tweak Regex compiler's FindFirstChar code gen (#2342)

Tweak Regex compiler's FindFirstChar code gen (#2342)

* Tweak Regex compiler's FindFirstChar code gen

- If we're able to use IndexOf{Any} and we don't have any information on subsequent characters, we can avoid spitting the code related to looping, avoid unnecessary slicing, etc.
- If the Boyer-Moore prefix contains non-ASCII text, we currently don't use it when compiling FindFirstChar, but we previously made a change to also skip computing other prefix information if we got a Boyer-Moore prefix, which ends up making FindFirstChar terrible when there's Unicode in the prefix string.  This fixes that to still compute the other prefix information in that case.
- We're currently often generating multiple "this.runtextpos = runtextend; return false" blocks.  We can consolidate them.
- Makes a few cleanliness changes to the assembly generator, e.g. ensuring the internal types are sealed, the types are all beforefieldinit as the C# compiler would do, etc.

Also added/tweaked a few tests.

* Address PR feedback