SSA simplify pass

[Henry Cejtin]

I  am pretty certain that reverse-sequential memory reads or writes are worse
than sequential memory reads or writes.  I don't know how much, and  I  don't
know  what  happens if the data is in the cache (L1 or L2).  I'm sure that it
varies with things like the mother-board chipset and the like.  I don't  know
how  much  effort  it is worth it given the uncertainty, but if it is easy, I
would say go for it.

I  just did a quick test, and the difference between pure forward reading and
pure backward reading was only about .3%.