[OpenCL] Fix __builtin_astype for vec3 types.

__builtin_astype does not generate correct LLVM IR for vec3 types. This patch inserts bitcasts to/from vec4 when necessary in addition to generating vector shuffle. Sema and codegen tests are added.

Differential Revision: http://reviews.llvm.org/D20133

llvm-svn: 272153