DataLayout: Fix latent issues with getMaxIndexSizeInBits (#118740)

Because it was implemented in terms of getMaxIndexSize, it was always
rounding the values up to a multiple of 8. Additionally, it was using
the PointerSpec's BitWidth rather than its IndexBitWidth, which was
self-evidently incorrect.

Since getMaxIndexSize was only used by getMaxIndexSizeInBits, and its
name and function seem niche and somewhat confusing, go ahead and remove
it until a concrete need for it arises.

llvm/include/llvm/IR/DataLayout.h

@@ -330,9 +330,6 @@ public:
   /// the backends/clients are updated.
   unsigned getPointerSize(unsigned AS = 0) const;
 
-  /// Returns the maximum index size over all address spaces.
-  unsigned getMaxIndexSize() const;
-
   // Index size in bytes used for address calculation,
   /// rounded up to a whole number of bytes.
   unsigned getIndexSize(unsigned AS) const;
@@ -369,9 +366,7 @@ public:
   }
 
   /// Returns the maximum index size over all address spaces.
-  unsigned getMaxIndexSizeInBits() const {
-    return getMaxIndexSize() * 8;
-  }
+  unsigned getMaxIndexSizeInBits() const;
 
   /// Size in bits of index used for address calculation in getelementptr.
   unsigned getIndexSizeInBits(unsigned AS) const {

llvm/lib/IR/DataLayout.cpp

@@ -740,12 +740,10 @@ unsigned DataLayout::getPointerSize(unsigned AS) const {
   return divideCeil(getPointerSpec(AS).BitWidth, 8);
 }
 
-unsigned DataLayout::getMaxIndexSize() const {
+unsigned DataLayout::getMaxIndexSizeInBits() const {
   unsigned MaxIndexSize = 0;
   for (const PointerSpec &Spec : PointerSpecs)
-    MaxIndexSize =
-        std::max(MaxIndexSize, (unsigned)divideCeil(Spec.BitWidth, 8));
-
+    MaxIndexSize = std::max(MaxIndexSize, Spec.IndexBitWidth);
   return MaxIndexSize;
 }