[SemaExpr] Re-enable missing assertion.

This has been disabled for a long time, but:
1) Initializers work (and apparently they're re reason why this was disabled).
2) various tests happen to hit this code path and the invariant seems to be
always verified.

Differential Revision:	http://reviews.llvm.org/D12110
Reviewed by:	rsmith

llvm-svn: 245404