Setting atomic reference to nil crashes the compiler

[ggiraldez]

The following code crashes the compiler:

class Foo
end

a = Atomic(Foo?).new nil

a.set Foo.new
a.set nil

The full error message is:

Module validation failed: Stored value type does not match pointer operand type!
  store atomic volatile %Nil zeroinitializer, %Foo** %0 seq_cst, align 8, !dbg !17
 %Foo*
0x10da5dc23: *raise<String>:NoReturn at ??
0x10e9ad3dc: *Crystal::CodeGenVisitor#finish:Nil at ??
0x10e8e7f7e: *Crystal::Compiler#codegen<Crystal::Program, Crystal::ASTNode+, Array(Crystal::Compiler::Source), String>:(Tuple(Array(Crystal::Compiler::CompilationUnit), Array(String)) | Nil) at ??
0x10f1364f4: *Crystal::Compiler#compile<Array(Crystal::Compiler::Source), String>:Crystal::Compiler::Result at ??
0x10f1d8019: *Crystal::Command#run_command<Bool>:Nil at ??
0x10dac3c71: *Crystal::Command#run:(Bool | Crystal::Compiler::Result | IO::FileDescriptor | Nil) at ??
0x10da8c62d: main at ??

Error: you've found a bug in the Crystal compiler. Please open an issue, including source code that will allow us to reproduce the bug: https://github.com/crystal-lang/crystal/issues

The error is triggered at the second set call.

Seen on Crystal 0.20.5 and 0.21.0.