[Bugfix] Fix ICE on constexpr vector splat.

In {CG,}ExprConstant.cpp, we weren't treating vector splats properly.
This patch makes us treat splats more properly.

Additionally, this patch adds a new cast kind which allows a bool->int
cast to result in -1 or 0, instead of 1 or 0 (for true and false,
respectively), so we can sanely model OpenCL bool->int casts in the AST.

Differential Revision: http://reviews.llvm.org/D14877


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@257559 91177308-0d34-0410-b5e6-96231b3b80d8