gallium/llvmpipe: encode TG4 component in sampler swizzle

Due to how llvmpipe encodes immediates in LLVM const vectors we have issues pulling this value back out at the right place in the stack, @sroland suggested we change the TG4 encoding to be like SM5 encoding, and put the constant integer component into the sampler src swizzleX. I've done this using a CAP to control it primarily because virgl would need some work to handle it, but also I'm not confident in moving other drivers over yet. It seems trivial for nouveau, r600 and radeonsi to move over if they want.