Use-and-clobber insns and some ancient combine.c code

[Richard Kenner]

    Your change added a *second* test, for substitution of

      (set (reg Y)  (...(some use of reg X)...))
    into
      (parallel [ ...(some use of reg Y)...  (clobber (reg X)) ...])

    It this *this* check I don't understand.

I think it's indeed the earlyclobber issue: you can't be sure that the
clobber will happen before X is read.