[WebAssembly] Fix call_indirect on funcrefs

The currently implementation of funcrefs is broken since it is putting
the funcref itself on the stack before the call_indirect. Instead what
should be on the stack is the constant 0, which is the index at which
we store the funcref in __funcref_call_table.

Reviewed By: tlively

Differential Revision: https://reviews.llvm.org/D111152