Small perf tweaks
Simplifying scanner code.
Remove unneeded method from parser.
EndOfFile tokens are reusable in incremental parsing.

Also, only regex and merged > tokens are unusable by the incremental parser.
Produce merged > tokens (like >>, >>=, etc.) in the scanner, not the parser.
Disable expensive asserts.
Updated comments.
Update LKG.
Simplify scanner code.
More accurate sourcemap generation.

Properly add source map record spans for the code we emit and the
code it was generated from.
Simplify emitter code.
Update baselines.