radeon/llvm: Add pseudo-support for 64-bit immediate types on SI

radeon/llvm: Add pseudo-support for 64-bit immediate types on SI

SI does not support 64-bit immediates natively, but llvm will generate
i64 immediates when indexing loads and stores (since SI has 64-bit
pointers).  The i64 indices will always be small enough to fit into
32-bits (i.e. the high 32 bits will always be all zeros), so we can
treat these index values as 32-bits.