R600/SI: add post ISel folding for SI v2

Include immediate folding and SGPR limit handling for VOP3 instructions.

v2: remove leftover hasExtraSrcRegAllocReq

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
llvm-svn: 176101