[GR-63729] SVM: Constant fold the object header when initializing an object

[graalvmbot]

Hi,

Object header is computed by the native image compiled code at runtime, on x86:

lea rax, [r14_heapbase + hub_offset]
sub rax, r14_heapbase
shl rax, 2
mov [rcx], eax

The reason we cannot constant fold this sequence is because the hub_offset is not really a constant at compile time, it is patched in at the last stage of image building. We can have a dedicated constant type for object header, and patch the value into the code stream, reducing the sequence into

mov DWORD PTR [rcx], hub_offset << 2

Please take a look and leave your reviews, thanks a lot.