Winch x64: panic compiling`i32.atomic.load16_u` with large offset

[alexcrichton]

This input:

(module
  (memory 2 )
  (func (export "f") (param i32) (result i32)
    local.get 0
    i32.atomic.load16_u offset=65536
  )
)

currently fails with:

$ cargo run compile -C compiler=winch foo.wat
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.17s
     Running `target/debug/wasmtime compile -C compiler=winch foo.wat`

thread '<unnamed>' (1303362) panicked at winch/codegen/src/isa/x64/asm.rs:1196:81:
called `Result::unwrap()` on an `Err` value: TryFromIntError(())
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

cc @saulecabrera

[alexcrichton]

A slight variant of this:

(module
  (memory 2)
  (func (export "f") (param i32) (result i32)
    local.get 0
    i32.atomic.load16_u offset=0x80000000
  )
)

panics with:

$ cargo run compile -C compiler=winch foo.wat
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.12s
     Running `/home/alex/code/wasmtime2/target/debug/wasmtime compile -C compiler=winch foo.wat`

thread '<unnamed>' (1320511) panicked at cranelift/codegen/src/isa/x64/inst/mod.rs:164:9:
assertion failed: dst_size.is_one_of(&[OperandSize::Size32, OperandSize::Size64])
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace