[SVE] Fix getAlignmentInfo for scalable vectors

[david-arm]

Summary

When calculating the natural alignment for scalable vectors it
is acceptable to calculate an allocation size based on the minimum
number of elements in the vector.

This code path is exercised by an existing test:

CodeGen/AArch64/sve-intrinsics-int-arith.ll

[efriedma]

LGTM

llvm/lib/IR/DataLayout.cpp

if (I != Alignments.begin()) {
       if (I->AlignType == INTEGER_ALIGN)
         return ABIInfo ? I->ABIAlign : I->PrefAlign;
     }
   } else if (AlignType == VECTOR_ALIGN) {
     // By default, use natural alignment for vector types. This is consistent
     // with what clang and llvm-gcc do.
     unsigned Alignment =
         getTypeAllocSize(cast<VectorType>(Ty)->getElementType());
-    Alignment *= cast<VectorType>(Ty)->getNumElements();
+    // We're only calculating a natural alignment, so it doesn't have to be
+    // based on the full size for scalable vectors. Using the minimum element
+    // count should be enough here.
+    Alignment *= cast<VectorType>(Ty)->getElementCount().Min;
     Alignment = PowerOf2Ceil(Alignment);
     return Align(Alignment);
    }
 
   // If we still couldn't find a reasonable default alignment, fall back
   // to a simple heuristic that the alignment is the first power of two
   // greater-or-equal to the store size of the type.  This is a reasonable
   // approximation of reality, and if the user wanted something less