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.
Tom Stellard
a488fdd3d9