Analysis of high priority PR c/2454

[Richard Kenner]

    The true issue is that LOAD_EXTEND_OP only applies to paradoxical
    subreg's whose arguments are MEMs, i.e. those that really require a
    load from memory.  Most uses of LOAD_EXTEND_OP check for the MEM, but
    a few including those in combine.c's num_sign_bit_copies and
    nonzero_bits don't.

But doesn't reload *force* a reload of paradoxical SUBREGs, so that they
*always* come from memory?  It *used* to, but perhaps that has changed.
If it still does, something is odd here.