Cherry-pick some changes for SIMD and WebAssembly#96

Merged

nikic merged 4 commits intorust-lang:rustc/12.0-2021-02-03from alexcrichton:wasm-simd-update

Mar 18, 2021

+854-1,647

Commits on Mar 18, 2021

[WebAssembly] Prototype i8x16 to i32x4 widening instructions

tlively
authored and
alexcrichton
committed
[WebAssembly] Remove unimplemented-simd target feature

tlively
authored and
alexcrichton
committed
[WebAssembly] Remove experimental SIMD instructions

tlively
authored and
alexcrichton
committed
[WebAssembly] Finalize SIMD names and opcodes

tlively
authored and
alexcrichton
committed