[dfsan] Fix origin tracking for fast8

The problem is the following. With fast8, we broke an important
invariant when loading shadows.  A wide shadow of 64 bits used to
correspond to 4 application bytes with fast16; so, generating a single
load was okay since those 4 application bytes would share a single
origin.  Now, using fast8, a wide shadow of 64 bits corresponds to 8
application bytes that should be backed by 2 origins (but we kept
generating just one).

Let’s say our wide shadow is 64-bit and consists of the following:
0xABCDEFGH. To check if we need the second origin value, we could do
the following (on the 64-bit wide shadow) case:

 - bitwise shift the wide shadow left by 32 bits (yielding 0xEFGH0000)
 - push the result along with the first origin load to the shadow/origin vectors
 - load the second 32-bit origin of the 64-bit wide shadow
 - push the wide shadow along with the second origin to the shadow/origin vectors.

The combineOrigins would then select the second origin if the wide
shadow ...