[WebAssembly] Do nothing when sinking to same place

In `DebugValueManager`, if a `Def` is requested to be sunk to the same
place (i.e., `Insert` is right after `Def`, not counting `DBG_VALUE`s)
currently we still do the sink. This can result in unnecessary creation
of `DBG_VALUE $noreg`. See comments for details. This CL detects this
case and do nothing and return, so we don't end up creating unnecessary
undef `DBG_VALUE`s.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D146860