relational: Implement shuffle2 builtin

This was added in CL 1.1

Tested with a Radeon HD 7850 (Pitcairn) using the CL CTS via:
test_conformance/relationals/test_relationals shuffle_built_in_dual_input

v2: Add half support to shuffle2
    Move shuffle2 to misc/

Signed-off-by: Aaron Watry <awatry@gmail.com>
Reviewed-by: Jan Vesely <jan.vesely@rutgers.edu>
llvm-svn: 312404