R600/SI: Store constant initializer data in constant memory

R600/SI: Store constant initializer data in constant memory

This implements a solution for constant initializers suggested
by Vadim Girlin, where we store the data after the shader code
and then use the S_GETPC instruction to compute its address.

This saves use the trouble of creating a new buffer for constant data
and then having to pass the pointer to the kernel via user SGPRs or the
input buffer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213530 91177308-0d34-0410-b5e6-96231b3b80d8