[OpenCL] Fix casting a true boolean to an integer vector.

OpenCL v1.1 s6.2.2: for the boolean value true, every bit in the result vector should be set.

This change treats the i1 value as signed for the purposes of performing the cast to integer,
and therefore sign extend into the result.

Patch by Neil Hickey!

http://reviews.llvm.org/D13349

llvm-svn: 249301