Handle tied operands that are not killed by their use.

Any tied register uses are interesting enough to be added to the reguses
list if their value is not killed.

A copy needs to be inserted in that case.

cranelift/filetests/regalloc/constraints.cton

@@ -15,6 +15,19 @@ ebb0:
     return v2
 }
 
+; Tied operand is live after instruction.
+function %tied_alive() -> i32 {
+ebb0:
+    v0 = iconst.i32 12
+    v1 = iconst.i32 13
+    ; check: $(v0c=$V) = copy $v0
+    ; check: $v2 = isub $v0c, $v1
+    v2 = isub v0, v1
+    ; check: $v3 = iadd $v2, $v0
+    v3 = iadd v2, v0
+    return v3
+}
+
 ; Fixed register constraint.
 function %fixed_op() -> i32 {
 ebb0: