Use `signext` and `zeroext` argument attributes.

[sunfishcode]

Rust should use LLVM's signext and zeroext argument attributes for bool/i8/i16/u8/u16 types when targetting WebAssembly, for consistency with LLVM IR generated by clang, and to allow these values to be automatically interpreted by JS.

For example, on this Rust code:

#[no_mangle]
pub extern fn narrow(x: i32) -> i8 {
  x as i8
}
#[no_mangle]
pub extern fn widen(x: i8) -> i32 {
  x as i32
}

Running rustc test.rs --emit llvm-ir --crate-type=cdylib --target=wasm32-unknown-unknown produces:

[...]
define i8 @narrow(i32 %x) unnamed_addr #0 {
[...]
define i32 @widen(i8 %x) unnamed_addr #0 {
[...]

It should have attributes like this:

[...]
define signext i8 @foo(i32 %x) unnamed_addr #0 {
[...]
define i32 @bar(i8 signext %x) unnamed_addr #0 {
[...]

[alexcrichton]

Oh nice catch! I've submitted rust-lang/rust#48959 to cover this

[alexcrichton]

Fixed in rust-lang/rust#48959