MergeSimilarFunctions optimization pass (#4414)

Merge similar functions that only differs constant values (like immediate
operand of const and call insts) by parameterization.
Performing this pass at post-link time can merge more functions across
objects. Inspired by Swift compiler's optimization which is derived from
LLVM's one:
https://github.com/apple/swift/blob/main/lib/LLVMPasses/LLVMMergeFunctions.cpp
https://github.com/llvm/llvm-project/blob/main/llvm/docs/MergeFunctions.rst

The basic ideas here are constant value parameterization and direct callee
parameterization by indirection.

Constant value parameterization is like below:

;;  Before
(func $big-const-42 (result i32)
  [[many instr 1]]
  (i32.const 44)
  [[many instr 2]]
)
(func $big-const-43 (result i32)
  [[many instr 1]]
  (i32.const 45)
  [[many instr 2]]
)

;; After
(func $byn$mgfn-shared$big-const-42 (result i32)
  [[many instr 1]]
  (local.get $0) ;; parameterized!!
  [[many instr 2]]
)
(func $big-const-42 (result i32)
  (call $byn$mgfn-shared$big-const-42
   (i32.const 42)
  )
)
(func $big-const-43 (result i32)
  (call $byn$mgfn-shared$big-const-42
   (i32.const 43)
  )
)

Direct callee parameterization is similar to the constant value parameterization,
but it parameterizes callee function i by ref.func instead. Therefore it is enabled
only when reference-types and typed-function-references features are enabled.

I saw 1 ~ 2 % reduction for SwiftWasm binary and Ruby's wasm port
using wasi-sdk, and 3 ~ 4.5% reduction for Unity WebGL binary when -Oz.

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