Binary encoding + decoding + text printing support for i32.atomic.load ordering immediate

[stevenfontanella]

• Binary encoding + decoding for atomic ordering immediate for i32.atomic.load
  • The decoding is tested by the spec test. Encoding was checked manually by encoding the same spec test, as well as side-by-side with Binaryen's implementation, and by round-tripping the spec test (see below).
• Text printing for ordering immediate for i32.atomic.load
  • Tested by round-tripping the spec test (see below).

Binary format output:

xxd out.wasm
00000000: 0061 736d 0100 0000 0184 8080 8000 0160  .asm...........`
00000010: 0000 0382 8080 8000 0100 0584 8080 8000  ................
00000020: 0101 0101 0a9e 8080 8000 0198 8080 8000  ................
00000030: 0041 00fe 1002 001a 4100 fe10 1201 001a  .A......A.......
00000040: 4100 fe10 0200 1a0b                      A.......

Explainer for the binary format output:

1. • 0xfe10 - i32.atomic.load
   • 0x02 - alignment of 2 (2^2 = 4 bytes)
   • 0x00 - offset of 0
2. • 0xf10 - i32.atomic.load
   • 12 - alignment of 2 | bit 4 set indicating that an ordering immediate follows
   • 01 - acqrel ordering
   • 0x00 - offset of 0

The third case is encoded the same as the first, since SeqCst ordering is the default and we choose not to emit it in the binary format.

Round-tripped WAST shows that the text + binary are round-tripped correctly (seqcst immediate is dropped in both cases since it's the default):

./interpreter/wasm test/core/relaxed-atomics/relaxed-atomics.wast -o out.wast

(module
  (type $0 (func))
  (memory $0 1 1)
  (func $0
    (type 0)
    (i32.const 0)
    (i32.atomic.load)
    (drop)
    (i32.const 0)
    (i32.atomic.load acqrel)
    (drop)
    (i32.const 0)
    (i32.atomic.load)
    (drop)
  )
)
(assert_malformed
  (module quote
    "(memory 1 1 shared) (func $i32load (drop (i32.load acqrel (i32.const 51))))"
  )
  "unexpected token"
)
(module
  (type $0 (func))
  (memory $0 1 1)
  (func $0
    (type 0)
    (i32.const 0)
    (i32.atomic.load)
    (drop)
    (i32.const 0)
    (i32.atomic.load acqrel)
    (drop)
    (i32.const 0)
    (i32.atomic.load)
    (drop)
  )
)

[tlively]

@conrad-watt, PTAL