Fix segfault when incrementing whole vector values.

Also removed RefExpr::Eval(Val*) method since it was never called
(Clang emitted warning about this hiding overloaded virtual function
UnaryExpr::Eval(Frame*)) and doesn't appear to be necessary even if it
was called to avoid the default vector handling of UnaryExpr::Eval
(as the comment suggests as the intention).

testing/btest/Baseline/language.incr-vec-expr/out

@@ -0,0 +1,5 @@
+[0, 0, 0]
+[a=0, b=test, c=[1, 2, 3]]
+[1, 1, 1]
+[a=1, b=test, c=[1, 2, 3]]
+[a=1, b=test, c=[2, 3, 4]]