Sometimes the spilling logic will decide to spill a merged def. This is fine
for 2- and 4-wide values as there are appropriate load/store instructions for
those, but not so with 3-wide values. We must take that into account in the
SpillCodeInserter and emit a 64-bit and 32-bit store into lmem (and similar for
load, although less important, as it tends to have gotten split up by the time
the use rolls around).

Unfortunately I've lost track of the program that repro'd this issue.