[llvm] Add default constructor of llvm::ElementCount.

This patch prevents failures like those reported in
http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast/builds/34173.

We have enabled the default constructor for
`llvm::ElementCount` to make sure the code compiles on Windows.

Reviewed By: ormris

Differential Revision: https://reviews.llvm.org/D86240

include/llvm/IR/Intrinsics.h

@@ -134,9 +134,7 @@ namespace Intrinsic {
       unsigned Pointer_AddressSpace;
       unsigned Struct_NumElements;
       unsigned Argument_Info;
-      // There is no default constructor in `ElementCount`, so we need
-      // to explicitly initialize this field with a value.
-      ElementCount Vector_Width = ElementCount::getFixed(0);
+      ElementCount Vector_Width;
     };
 
     enum ArgKind {

include/llvm/Support/TypeSize.h

@@ -35,14 +35,12 @@ private:
   ElementCount(unsigned Min, bool Scalable) : Min(Min), Scalable(Scalable) {}
 
 public:
-  /// No default constructor. Users should use one of the `get*`
-  /// static methods below, as they should always make a conscious
-  /// choice on the type of `ElementCount` they are requesting.
-  ElementCount() = delete;
   unsigned Min;  // Minimum number of vector elements.
   bool Scalable; // If true, NumElements is a multiple of 'Min' determined
                  // at runtime rather than compile time.
 
+  ElementCount() = default;
+
   ElementCount operator*(unsigned RHS) {
     return { Min * RHS, Scalable };
   }