[WebAssembly] Add SIMD intrinsics using unsigned integers

For each SIMD intrinsic function that takes or returns a scalar signed integer
value, ensure there is a corresponding intrinsic that returns or an
unsigned value. This is a convenience for users who use -Wsign-conversion so
they don't have to insert explicit casts, especially when the intrinsic
arguments are integer literals that fit into the unsigned integer type but not
the signed type.

Differential Revision: https://reviews.llvm.org/D108412