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