[WebAssembly] Suffix output operands with '='.

[WebAssembly] Suffix output operands with '='.

This distinguishes input operands from output operands. This is something of
a syntactic experiment to see whether the mild amount of clutter this adds is
outweighed by the extra information it conveys to the reader.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253922 91177308-0d34-0410-b5e6-96231b3b80d8