Instructions defined in the original inner loop preheader may depend on
values defined in the outer loop header, but the inner loop header will
become the entry block in the loop nest. Move the instructions from the
preheader to the outer loop header, so we do not break dominance. We
also have to check for unsafe instructions in the preheader. If there
are no unsafe instructions, all instructions should be movable.
Currently we move all instructions except the terminator and rely on
LICM to hoist out invariant instructions later.
Fixes PR45743
Values defined in the outer loop header could be used in the inner loop
latch. In that case, we need to create LCSSA phis for them, because after
interchanging they will be defined in the new inner loop and used in the
new outer loop.
Florian Hahn