Since the shuffle mask is not exposed as an operand in the native ISel
DAG, create a new WebAssembly ISD node exposing the mask. The mask is
lowered as sixteen immediate byte indices no matter what type the
original vector shuffle was operating on.
This CL depends on D51656
Maybe it's better to insert an if statement to check if the size of Mask array matches the vector's operand type so that it can safely be converted to v128 vector operations. For example, in case we have two 4xi32 vectors but the size of this Mask array is not equal to 4, the instruction selection should fail.