Reapply "R600: Add new intrinsic to read work dimensions"

This effectively reverts revert 219707. After fixing the test to work with
new function name format and renamed intrinsic.

Reviewed-by: Tom Stellard <tom@stellard.net>
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219710 91177308-0d34-0410-b5e6-96231b3b80d8