Replace the OnStack boolean in VarLoc with a flag bit in StackOffset.
This doesn't get much memory savings since it's still 64-bit aligned,
but does seem to help a bit anyway.
Change liveSlot to fit into 16 bytes. Because nested structs still get
padding, this required inlining it. Fortunately there's not much logic
to copy.
Change-Id: Ie19a409daa41aa310275c4517a021eecf8886441
Reviewed-on: https://go-review.googlesource.com/92401 Reviewed-by: David Chase <drchase@google.com>