[WebAssembly] Only RAUW a constant once in FixFunctionBitcasts

When we collect 2 uses of a function in FindUses and then RAUW when we
visit the first, we end up visiting the wrapper (because the second was
RAUW'd).  We still want to use RAUW instead of just Use->set() because
it has special handling for Constants, so this patch just ensures that
only one use of each constant is added to the work list.

Differential Revision: https://reviews.llvm.org/D28504

llvm-svn: 291603