chore(doc): document for immutable_load_eliminate (#130)

Author: HerrCai0907

docs/opt/immutable_load_eliminate.md

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+During coding, there are always lots of never changed static data which is similar as .text section in ELF.
+
+In AS, this type of immutable data mainly includes string literals and function object.
+
+## Function Object
+
+For function objects, if we can identify this kinds of data, we can simplify lots of indirect function call.
+
+Considering below example for function object:
+
+```ts
+function v(fn: () => void): void {
+  fn();
+}
+v(() => {});
+```
+
+Arrow function `() => {}` will be compiled as function object which stored in static data area with function index `I`.
+The function `v` will accept a `ptr` which is pointing to the function object. During calling `fn`, the function index will be load from `ptr` and call arrow function by `call_indirect`.
+
+After eliminate the load data from immutable data area, subsequence compilation optimization can infer the function index in `call_indirect` is never changed and simplify it as `call`.
+
+## String Literal
+
+not implement yet.
+
+## How
+
+Binaryen cannot optimize this cases directly because this semantics cannot be properly expressed in a wasm spec.
+
+The pass rely on the additional information `ImmutableDataElementRanges` passed in `AsModule` which define the ranges of immutable data.
+
+When load data inside this range, the load instruction will be replaced as const instruction.