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

[Robbepop]

(module
  (func (export "test") (param i32) (result f32)
    (` (local.get 0))
    (if
        (i32.eqz (local.get 0))
        (then
            (local.get 0)
            (f32.convert_i32_u)
            (drop)
        )
    )
  )
)

Stitch returns 0 for test(0) and test(1) while it should return 0 and 1 respectively. The reason likely is that the register on the stack from f32.convert_i32_s is not preserved prior to entering if.

[Robbepop]

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