Lost stack values when entering a loop

[gtrepta]

(module
  ;; Iterative factorial without locals.
  (func $pick0 (param i64) (result i64 i64)
    (local.get 0) (local.get 0)
  )
  (func $pick1 (param i64 i64) (result i64 i64 i64)
    (local.get 0) (local.get 1) (local.get 0)
  )
  (func (export "fac-ssa") (param i64) (result i64)
    (i64.const 1) (local.get 0)
    (loop $l (param i64 i64) (result i64)
      (call $pick1) (call $pick1) (i64.mul)
      (call $pick1) (i64.const 1) (i64.sub)
      (call $pick0) (i64.const 0) (i64.gt_u)
      (br_if $l)
      (drop) (return)
    )
  )
)

(assert_return (invoke "fac-ssa" (i64.const 25)) (i64.const 7034535277573963776))

After fac-ssa gets invoked and the loop instruction is reached, the two values that have been pushed onto the stack disappear when we enter the loop, so then the subsequent call to $pick1 doesn't have any parameters to use.

The spec

Our rule:

wasm-semantics/pykwasm/src/pykwasm/kdist/wasm-semantics/wasm.md

Lines 549 to 552 in ad238ca

	syntax Instr ::= #loop(VecType, Instrs, BlockMetaData) [symbol(aLoop)]
	// ------------------------------------------------------------------------------
	rule <instrs> #loop(VECTYP, IS, BLOCKMETA) => sequenceInstrs(IS) ~> label VECTYP { #loop(VECTYP, IS, BLOCKMETA) } VALSTACK ... </instrs>
	<valstack> VALSTACK => .ValStack </valstack>

It looks like the spec states that the stack values are prepended to the instructions when entering the block, but we aren't doing that here.