Unnecessary stack checks and prologue for wasm functions containing a single `return_call_ref`

[alexcrichton]

Given this input:

(module
  (type $t (func))
  (global (export "g") (mut (ref $t)) (ref.func $f))
  (func $f)

  (func (export "call")
    (return_call_ref $t (global.get 0))
  )
)

this currently compiles as:

$ wasmtime compile foo.wat
$ wasmtime objdump foo.cwasm --filter 1
wasm[0]::function[1]:
            pushq   %rbp
            movq    %rsp, %rbp
            movq    8(%rdi), %r10
            movq    0x18(%r10), %r10
            addq    $0x10, %r10
            cmpq    %rsp, %r10
            ja      0x52
            movq    0x30(%rdi), %rsi
            movq    %rdi, %r8
            movq    8(%rsi), %r10
            ╰─╼ trap: Normal(NullReference)
            movq    0x18(%rsi), %rdi
            movq    %r8, %rsi
            movq    %rbp, %rsp
            popq    %rbp
            jmpq    *%r10
            ud2
            ╰─╼ trap: Normal(StackOverflow)

There's a few inefficiencies with this codegen we could improve upon:

• There's an explicit stack check as cmpq %rsp, %r10 here which isn't necessary. That's injected for functions in general which call another function, but it's not injected for leaf functions, and in this case this is sort of a leaf function.
• This sets up a prologue/epilogue which isn't necessary for this "mostly leaf" function.
• There's a trap registered for a null function reference, but this function reference type can't be null so that's not necessary.

Nothing major here, but might be nice to clean up some of this as well.