Fixes two minor bugs that I came across in the old
CgCase code generation:

1. We were generating
	tmp = Sp[1]
	... more uses of Sp[1]....
instead of
	tmp = Sp[1]
	... more uses of tmp....
in the (case v of ...prim alts...) situation

2. The cost-centre restoration wasn't right for let-no-escapes

I kept this fix separate, becuase it does change the code generated
slightly.