Make srem have the sign of the dividend.

This is how remainder is defined in C (as of C99), C++ (as of C++11), Rust,
and WebAssembly, for example.

lib/cretonne/meta/base/instructions.py

@@ -683,14 +683,13 @@ urem = Instruction(
 
 srem = Instruction(
         'srem', """
-        Signed integer remainder.
+        Signed integer remainder. The result has the sign of the dividend.
 
         This operation traps if the divisor is zero.
 
         .. todo:: Integer remainder vs modulus.
 
-            Clarify whether the result has the sign of the divisor or the
-            dividend. Should we add a ``smod`` instruction for the case where
+            Should we add a ``smod`` instruction for the case where
             the result has the same sign as the divisor?
         """,
         ins=(x, y), outs=a, can_trap=True)