[WebAssembly] Undef invalid DBG_VALUEs after RegColoring

After register coalescing, some `DBG_VALUE`s can have incorrect info.
For example, if a `DBG_VALUE` has a register operand `%0`, but it
resides in a live range of `%1`, it has incorrect info after `%0` and
`%1` are coalesced. See the comments for more details.

This does not have meaningful changes on our variable debug info
coverage or compilation time, which is good news.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D151125