[SVE] Fix getAlignmentInfo for scalable vectors

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

Differential Revision: https://reviews.llvm.org/D79475

Author: david-arm

llvm/lib/IR/DataLayout.cpp

@@ -559,7 +559,10 @@ Align DataLayout::getAlignmentInfo(AlignTypeEnum AlignType, uint32_t BitWidth,
     // 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);
    }