[WebAssembly] Support opaque pointers in FixFunctionBitcasts

With opaque pointers, no actual bitcasts will be present. Instead,
there will be a mismatch between the call FunctionType and the
function ValueType. Change the code to collect CallBases
specifically (rather than general Uses) and compare these types.

RAUW is no longer performed, as there would no longer be any
bitcasts that can be RAUWd.

Differential Revision: https://reviews.llvm.org/D108880