[OptimizeForJS] Avoid 64-bit integer storage if it's possible

[MaxGraey]

i64.store(x, C)   =>   f64.store(x, C'),   
     where C' <- reinterpret<f64>(C),
           C' != NaN && !is_subnormal(C')

it will lower to one HEAP64[ptr] = C' instead two writes to HEAP32

Fix: emscripten-core/emscripten#13365

[MaxGraey]

Fuzzer found this edge case:

!
Wasm2JS (before/after) comparison error, expected to have '[fuzz-exec] calling func_48
[fuzz-exec] note result: func_48 => -6996475.0
[fuzz-exec] calling func_48_invoker
[LoggingExternalInterface logging 1719783932]
[fuzz-exec] calling func_49_invoker
[LoggingExternalInterface logging 1719783932]' == '[fuzz-exec] calling func_48
[fuzz-exec] note result: func_48 => -6996475.0
[fuzz-exec] calling func_48_invoker
[LoggingExternalInterface logging 1359052027]
[fuzz-exec] calling func_49_invoker
[LoggingExternalInterface logging 1359052027]', diff:

--- expected

+++ actual

@@ -1,6 +1,6 @@

 [fuzz-exec] calling func_48
 [fuzz-exec] note result: func_48 => -6996475.0
 [fuzz-exec] calling func_48_invoker
-[LoggingExternalInterface logging 1719783932]
+[LoggingExternalInterface logging 1359052027]
 [fuzz-exec] calling func_49_invoker
-[LoggingExternalInterface logging 1719783932]
+[LoggingExternalInterface logging 1359052027]

================================================================================

seed: 1387706149137627325

And reduction example looks like this:

(module
 (type $i32_=>_none (func (param i32)))
 (type $none_=>_none (func))
 (type $none_=>_i32 (func (result i32)))
 (import "fuzzing-support" "log-i32" (func $fimport$0 (param i32)))
 (memory $0 16 17)
 (export "func_46" (func $291))
 (export "func_49_invoker" (func $74))
 (func $74
  (if
   (i64.eqz
    (i64.load16_u align=1
     (i32.const 4)
    )
   )
   (call $fimport$0
    (i32.const -59)
   )
  )
 )
 (func $291 (result i32)
  (i64.store
   (i32.const 4)
   (i64.const 126)
  )
  (i32.const 0)
 )
)

However 126 should be valid 6.23e-322. But fuzzer can't interpret this right. Hmm... May be we should avoid subnormals

[MaxGraey]

it seems we should skip subnormals as well sue to non-determinism on some platforms

[MaxGraey]

Fuzzed: ITERATION: 50171