translate: deal with size overflows by casting to ptrdiff_t

This was discovered as a result of the draw-elements-base-vertex-neg
piglit test, which passes very negative offsets in, followed up by large
indices. The nouveau code correctly adjusts the pointer, but the
translate code needs to do the proper inverse correction. Similarly fix
up the SSE code to do a 64-bit multiply to compute the proper offset.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>