swift234

[loop-canonicalization] Change pre-header insertion so that it always occurs.

Previously, if a header had multiple predecessors or if the predecessor was not
a cond_br, we would not insert a pre-header. Now we always insert pre-headers.

This is important in terms of ensuring that ARC Sequence Opts is correct when it
inserts retains before loops.

Reviewed by Andy Trick.

rdar://23853221
SR-102