Create value aliases when necessary.

If a secondary value in the source pattern becomes a primary value in
the destination pattern, it is not possible to overwrite the definition
of the source value.

Instead, change the original source value to an alias to the new promary
value.

filetests/isa/riscv/expand-i32.cton

@@ -16,3 +16,6 @@ ebb0(v1: i32, v2: i32):
 }
 ; check: $v3 = iadd $v1, $v2
 ; check: $(cout=$V) = icmp ult, $v3, $v1
+; It's possible the legalizer will rewrite these value aliases in the future.
+; check: $v4 -> $cout
+; check: return $v3, $v4

lib/cretonne/meta/gen_legalizer.py

@@ -106,6 +106,7 @@ def emit_dst_inst(node, fmt):
     # type: (Def, Formatter) -> None
     exact_replace = False
     replaced_inst = None  # type: str
+    fixup_first_result = False
     if len(node.defs) == 0:
         # This node doesn't define any values, so just insert the new
         # instruction.
@@ -125,6 +126,7 @@ def emit_dst_inst(node, fmt):
             # Insert a new instruction since its primary def doesn't match the
             # src.
             builder = 'let {} = dfg.ins(pos)'.format(wrap_tup(node.defs))
+            fixup_first_result = node.defs[0].is_output()
 
     fmt.line('{}.{};'.format(builder, node.expr.rust_builder()))
 
@@ -139,6 +141,14 @@ def emit_dst_inst(node, fmt):
     if exact_replace:
         fmt.comment('exactreplacement')
 
+    # Fix up any output vars.
+    if fixup_first_result:
+        # The first result of the instruction just inserted is an output var,
+        # but it was not a primary result in the source pattern.
+        # We need to change the original value to an alias of the primary one
+        # we just inserted.
+        fmt.line('dfg.change_to_alias(src_{0}, {0});'.format(node.defs[0]))
+
 
 def gen_xform(xform, fmt):
     # type: (XForm, Formatter) -> None