[WebAssembly] Fix memory bug introduced in 52861809994c

[tlively]

Summary

The instruction at DefI can sometimes be destroyed by
rematerializeCheapDef, so it should not be used after calling that
function. The fix is to use Insert instead when examining additional
multivalue stackifications. Insert is the address of the new
defining instruction after all moves and rematerializations have taken
place.

llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp

for (auto MII = MBB.rbegin(); MII != MBB.rend(); ++MII) {
             Commuting.maybeCommute(Insert, TreeWalker, TII);
           // Proceed to the next operand.
           continue;
         }
 
         // Stackifying a multivalue def may unlock in-place stackification of
         // subsequent defs. TODO: Handle the case where the consecutive uses are
         // not all in the same instruction.
-        auto *SubsequentDef = DefI->defs().begin();
+        auto *SubsequentDef = Insert->defs().begin();
         auto *SubsequentUse = &Use;
-        while (SubsequentDef != DefI->defs().end() &&
+        while (SubsequentDef != Insert->defs().end() &&
                SubsequentUse != Use.getParent()->uses().end()) {
           if (!SubsequentDef->isReg() || !SubsequentUse->isReg())
             break;
           unsigned DefReg = SubsequentDef->getReg();
           unsigned UseReg = SubsequentUse->getReg();
           // TODO: This single-use restriction could be relaxed by using tees
           if (DefReg != UseReg || !MRI.hasOneUse(DefReg))
             break;