[WebAssembly] Handle more types of uses in WebAssemblyAddMissingPrototypes

Previously we were only handling bitcast operations, however
prototypeless functions can also appear in other places such as
comparisons and as function params.

Switch to using replaceAllUsesWith() to replace the prototype-less
function uses.  This new approach results in some redundant bitcasting
but is much simpler and handles all cases.

Differential Revision: https://reviews.llvm.org/D56938

llvm-svn: 352445