Use foreach loops for StructType::elements(). NFC.

Use foreach loops for StructType::elements().  NFC.

We had a few places where we did

for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {

but those could instead do

for (auto *EltTy : STy->elements()) {

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243136 91177308-0d34-0410-b5e6-96231b3b80d8