radeon/llvm: improve cube map handling

Add support for TEX2, TXB2, TXL2, fix SHADOWCUBE

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>
4 files changed