[WebAssembly] Fix a bug in MachineBasicBlock::findDebugLoc() call

Summary:
InsertPos is within the bacic block `Header`, so `findDebugLoc()` should
be called on not `MBB` but `Header` instead.

Reviewers: yurydelendik

Subscribers: jfb, dschuff, aprantl, sbc100, jgravelle-google, sunfish, JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D45648

llvm-svn: 330079