Fix assertion failure in global-merge with unused ConstantExpr

[olista01]

Summary

The global-merge pass was crashing because it assumes that all ConstantExprs (reached via the global variables that they use) have at least one user.

I haven't worked out a way to test this, as an unused ConstantExpr cannot be represented by serialised IR, and global-merge can only be run in llc, which does not run any passes which can make a ConstantExpr dead.

This (reduced to the point of silliness) C code triggers this bug when compiled for arm-none-eabi at -O1:

static a = 7;
static volatile b[10] = {&a};

c;
main() {
  c = 0;
  for (; c < 10;)
    printf(b[c]);
}

[ab]

Nice find, LGTM!

lib/CodeGen/GlobalMerge.cpp

for (size_t GI = 0, GE = Globals.size(); GI != GE; ++GI) {
     size_t CurGVOnlySetIdx = 0;
 
     // For each global, look at all its Uses.
     for (auto &U : GV->uses()) {
       // This Use might be a ConstantExpr.  We're interested in Instruction
       // users, so look through ConstantExpr...
       Use *UI, *UE;
       if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U.getUser())) {
+        if (CE->use_empty())
+          continue;
         UI = &*CE->use_begin();
         UE = nullptr;
       } else if (isa<Instruction>(U.getUser())) {
         UI = &U;
         UE = UI->getNext();
       } else {
         continue;
       }