Fix Extended Asm ignored constraints

Fix Extended Asm ignored constraints

This commit fixes the case where the register of for the Extended Asm
input or output is known. Before this commit, the following case:

  register long __a0 asm ("a0") = one;
  asm volatile (
       "ecall\n\t"
       : "+r" (__a0) // NOTE the +r here
  );

Didn't treat `a0` as an input+output register (+ contraint) as the code
skipped the constraint processing when the register was already chosen
(instead of allocated later).

This issue comes from f081acbfba84ffdf1e479f932906bf10f88cd1c2, that was
taken as a reference in every other Extended Assembler implementation.