[WebAssembly] Improve WasmSignatureDenseMapInfo.

Let X and Y be types. Previously, functions F(X, Y) and G(Y, X) had
the same hash value because their hash values are computed as follows:

  hash(F) = hash(X) + hash(Y)
  hash(G) = hash(Y) + hash(X)

This patch fixes the issue by using hash_combine.

Differential Revision: https://reviews.llvm.org/D43856

llvm-svn: 326336