codegen: type T_intN more precisely as IntegerType pointers.
bitshifts: don't do checked conversion to Int32.

We should maybe not even force this to go through Int32 since that's
not what the LLVM instructions need anyway. I worry that this may
cause weird extra ops that aren't really necessary.
ByteVec: fast cmp implementation.

This took some careful tweaking, but I managed to get this to generate
the same code I was trying to get an intrinsic to produce. This makes
me wonder if I shouldn't try to do the same thing with bytevec_eq and
may some of the other bytevec intrinsics.