[WebAssembly] Use MachineInstr::setDebugValueUndef

When making `DBG_VALUE`/`DBG_VALUE_LIST` instructions undefined, there
is a method that takes care of it so we don't need to do it manually.
This changes the test because previously we are converting
`DBG_VALUE_LIST`s into `DBG_VALUE $noreg` but now we leave
`DBG_VALUE_LIST` but set it to undef by turning all its register
operands `$noreg`. The effect is the same.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D145998