`makepad-stitch` should preserve live locals upon entering control flow

[Robbepop]

The makepad-stitch interpreter evaluates the following .wast script incorrectly:

(module
  (func (export "run") (param $cond i32) (param $x i32) (result i32)
    (drop
      (i32.add
        (local.get $x)
        (i32.const 10)
      )
    )
    (local.get $x)
    (block
      (local.get $cond)
      (br_if 0)
      (local.set $x (i32.const 1))
    )
  )
)

(assert_return
    (invoke "run" (i32.const 1) (i32.const 42))
    (i32.const 42)
)

What happens:

• (local.get $x) puts $x on the stack.
• (br_if 0) breaks out of block before (local.set $x (i32.const 1)) since $cond is 1
• thus it is expected that $x is still on the stack. however, makepad-stitch always just returns 0.
• for $cond = 0 makepad-stitch actually returns $x back.

I have looked into makepad-stitch's code and think it might have to do with the fact that not all local variables on the stack are preserved (copied to temp stack slots) when entering a control frame such as block, if and loop.

I tested this manually with makepad-stitch's CLI tool installed via cargo install makepad-stitch.

cc @ejpbruel2

[Robbepop]

Some .wast tests from the Wasmi testsuite that could help here:
https://github.com/wasmi-labs/wasmi-tests/blob/main/tests/preserve-locals.wast