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

[fpetrogalli]

Summary

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.

[fpetrogalli]

I re-enabled the default contructor so that the code actually builds on linux.

[ormris]

This builds on Windows. Do the comments need to be updated?

[fpetrogalli]

In D86240#2227031, @ormris wrote:

This builds on Windows. Do the comments need to be updated?

Done. I'll submit this as soon as you give me a formal OK.

Thanks!

Francesco

[ormris]

LGTM

llvm/include/llvm/IR/Intrinsics.h

struct IITDescriptor {
     } Kind;
 
     union {
       unsigned Integer_Width;
       unsigned Float_Width;
       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 {
       AK_Any,
       AK_AnyInteger,
       AK_AnyFloat,
       AK_AnyVector,
       AK_AnyPointer,

llvm/include/llvm/Support/TypeSize.h

private:
   /// Prevent code from using initializer-list contructors like
   /// ElementCount EC = {<unsigned>, <bool>}. The static `get*`
   /// methods below are preferred, as users should always make a
   /// conscious choice on the type of `ElementCount` they are
   /// requesting.
   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 };
   }
   ElementCount operator/(unsigned RHS) {
     assert(Min % RHS == 0 && "Min is not a multiple of RHS.");
     return { Min / RHS, Scalable };
   }