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 f081acb, that was
taken as a reference in every other Extended Assembler implementation.

arm-asm.c

@@ -3075,7 +3075,6 @@ ST_FUNC void asm_compute_constraints(ASMOperand *operands,
                 tcc_error
                     ("asm regvar requests register that's taken already");
             reg = op->reg;
-            goto reg_found;
         }
       try_next:
         c = *str++;
@@ -3095,7 +3094,9 @@ ST_FUNC void asm_compute_constraints(ASMOperand *operands,
         case 'r': // general-purpose register
         case 'p': // loadable/storable address
             /* any general register */
-            for (reg = 0; reg <= 8; reg++) {
+            if ((reg = op->reg) >= 0)
+                goto reg_found;
+            else for (reg = 0; reg <= 8; reg++) {
                 if (!is_reg_allocated(reg))
                     goto reg_found;
             }

i386-asm.c

@@ -1342,7 +1342,6 @@ ST_FUNC void asm_compute_constraints(ASMOperand *operands,
 	    if (is_reg_allocated(op->reg))
 	        tcc_error("asm regvar requests register that's taken already");
 	    reg = op->reg;
-	    goto reg_found;
 	}
     try_next:
         c = *str++;
@@ -1385,12 +1384,17 @@ ST_FUNC void asm_compute_constraints(ASMOperand *operands,
         case 'D':
             reg = 7;
         alloc_reg:
+            if (op->reg >= 0 && reg != op->reg)
+                goto try_next;
             if (is_reg_allocated(reg))
                 goto try_next;
             goto reg_found;
         case 'q':
             /* eax, ebx, ecx or edx */
-            for(reg = 0; reg < 4; reg++) {
+            if (op->reg >= 0) {
+                if ((reg = op->reg) < 4)
+                    goto reg_found;
+            } else for(reg = 0; reg < 4; reg++) {
                 if (!is_reg_allocated(reg))
                     goto reg_found;
             }
@@ -1399,7 +1403,9 @@ ST_FUNC void asm_compute_constraints(ASMOperand *operands,
 	case 'R':
 	case 'p': /* A general address, for x86(64) any register is acceptable*/
             /* any general register */
-            for(reg = 0; reg < 8; reg++) {
+            if ((reg = op->reg) >= 0)
+                goto reg_found;
+            else for(reg = 0; reg < 8; reg++) {
                 if (!is_reg_allocated(reg))
                     goto reg_found;
             }

riscv64-asm.c

@@ -1720,7 +1720,6 @@ ST_FUNC void asm_compute_constraints(ASMOperand *operands,
                 tcc_error
                     ("asm regvar requests register that's taken already");
             reg = op->reg;
-            goto reg_found;
         }
       try_next:
         c = *str++;
@@ -1739,7 +1738,9 @@ ST_FUNC void asm_compute_constraints(ASMOperand *operands,
         case 'p': // loadable/storable address
             /* any general register */
             /* From a0 to a7 */
-            for (reg = 10; reg <= 18; reg++) {
+            if ((reg = op->reg) >= 0)
+                goto reg_found;
+            else for (reg = 10; reg <= 18; reg++) {
                 if (!is_reg_allocated(reg))
                     goto reg_found;
             }
@@ -1753,7 +1754,9 @@ ST_FUNC void asm_compute_constraints(ASMOperand *operands,
         case 'f': // floating pont register
             /* floating point register */
             /* From fa0 to fa7 */
-            for (reg = 42; reg <= 50; reg++) {
+            if ((reg = op->reg) >= 0)
+                goto reg_found;
+            else for (reg = 42; reg <= 50; reg++) {
                 if (!is_reg_allocated(reg))
                     goto reg_found;
             }