better field access of types which have one possible value

* When you do field access of a type which only has one possible
   value, the result is comptime-known.
 * StorePtr instructions which operate on pointers to types which
   only have one possible value, the result is a comptime no-op.

closes #1554
cbce61a209Andrew Kelley committed on 2/21/2019, 9:09:14 PMparent1066004
2 files changedLine totals unavailable