[GlobalMerge] Use vector::assign in place of fill+resize. NFC (llvm#85723)

Noticed while reviewing the code.

If the resize causes a new allocation, this will fill the new allocation
with zeroes directly. Previously, we would fill the old allocation with
zeroes, then copy them to the new allocation before filling the
additional space with zeros.

Author: topperc

llvm/lib/CodeGen/GlobalMerge.cpp

@@ -309,10 +309,9 @@ bool GlobalMergeImpl::doMerge(SmallVectorImpl<GlobalVariable *> &Globals,
   for (size_t GI = 0, GE = Globals.size(); GI != GE; ++GI) {
     GlobalVariable *GV = Globals[GI];
 
-    // Reset the encountered sets for this global...
-    std::fill(EncounteredUGS.begin(), EncounteredUGS.end(), 0);
-    // ...and grow it in case we created new sets for the previous global.
-    EncounteredUGS.resize(UsedGlobalSets.size());
+    // Reset the encountered sets for this global and grow it in case we created
+    // new sets for the previous global.
+    EncounteredUGS.assign(UsedGlobalSets.size(), 0);
 
     // We might need to create a set that only consists of the current global.
     // Keep track of its index into UsedGlobalSets.