In OpenCL, conversions between different vector types are disallowed

OpenCL 6.2.1 says: "Implicit conversions between built-in vector data types are
disallowed."  OpenCL 6.2.2 says: "Explicit casts between vector types are not
legal."  For example:

uint4 u = (uint4)(1);
int4 i = u; // invalid implicit conversion
int4 e = (int4)u; // invalid explicit conversion

Fixes PR10967. Submitted by: Anton Lokhmotov <Anton.lokhmotov@gmail.com>

llvm-svn: 140300