[WebAssembly] Print DEBUG_VALUE once for target indices

`DEBUG_VALUE` comments are printed before an instruction, so they are
not printed with `AddComment` method as other comments are, but printed
using `emitRawComment` method. But currently `emitDebugValueComment`
calls `emitRawComment` twice for target-index-based `DBG_VALUE`s: once
in the `switch`-`case`,
https://github.com/llvm/llvm-project/blob/d77ae7f2513504655e555cd326208598093d66e2/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp#L1192-L1193
and again at the end of the method:
https://github.com/llvm/llvm-project/blob/d77ae7f2513504655e555cd326208598093d66e2/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp#L1227-L1228

This makes them printed twice. I think this happened through multiple
commits modifying and refactoring this method.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D139579