Some drivers don't like constants constructed with OpUndef scalar components

[dneto0]

Example input:

void kernel foo(global float4* A)
{
  float4 value;
  value.w = 1111.0f;
  *A = value;
}

Produces assembly including this:

         %13 = OpUndef %float
 %float_1111 = OpConstant %float 1111
         %15 = OpConstantComposite %v4float %13 %13 %13 %float_1111
         %16 = OpSpecConstant %uint 1
         %17 = OpSpecConstant %uint 1
         %18 = OpSpecConstant %uint 1
%gl_WorkGroupSize = OpSpecConstantComposite %v3uint %16 %17 %18
         %20 = OpVariable %_ptr_Private_v3uint Private %gl_WorkGroupSize
         %21 = OpVariable %_ptr_StorageBuffer__struct_5 StorageBuffer
         %22 = OpFunction %void None %9
         %23 = OpLabel
         %24 = OpAccessChain %_ptr_StorageBuffer_v4float %21 %uint_0 %uint_0
               OpStore %24 %15
               OpReturn
               OpFunctionEnd

Some drivers have trouble with the OpConstantComposite. They happen to work if we use
%13 = OpConstantNull %float
instead.

[dneto0]

Use -hack-undef to work around this problem.

[dneto0]

This is solved well enough now.