[OpenCL] Remove extra select functions from opencl-c.h

OpenCL spec v2.0 s6.13.6:

gentype select (gentype a,
gentype b,
igentype c)

gentype select (gentype a,
gentype b,
ugentype c)

igentype and ugentype must have the same number
of elements and bits as gentype.

Differential Revision: https://reviews.llvm.org/D36259

llvm-svn: 310160