Restore registers appropriately in early returns

[hikalium]

$ ./compilium
int main() {
  int a = 1;
  return a + 2;
}
Preprocess begin
Parse begin
[
 FuncDef <main> : function<returns: int, args: []>{
  [
   (op=<> R=(op= R=(op=<a>))),
   (op=<return> R=(op=<+> L=(op=<a>) R=(op=<2>)))
  ]
 }
]

Analyze begin
`a` has a type: int
[
 FuncDef <main> : function<returns: int, args: []>{
  [
   (op=<> R=(op= R=(op=<a>))),
   (op=<return> R=(op=<+>:int reg: 1 L=(op=<a>:lvalue<int> reg: 1) R=(op=<2>:int reg: 2)))
  ]
 }
]

.intel_syntax noprefix
.text
.global _main
_main:
push rbp
mov rbp, rsp
push r12
push r13
push r14
push r15
lea rdi, [rbp - 4]
mov rsi, 1
mov [rdi], esi # 4 byte store
lea rdi, [rbp - 4]
movsxd rdi, dword ptr[rdi]
mov rsi, 2
add rdi, rsi
mov rax, rdi
mov rsp, rbp
pop rbp
ret
pop r15
pop r14
pop r13
pop r12
mov rsp, rbp
pop rbp
ret
.data

[hikalium]

pop r15
pop r14
pop r13
pop r12

should also be done in early returns.