Upgrade getModRefInfo to use our full gamut of information
Fix broken optimistic edge check and improve reporting when instructions are undefined due to cached PBs
Fix the order of operations when dealing with ignored loops so we never mistake a block we haven't got to yet for a dead one
Permit combining nulls and function pointers (considered scalars) in the same PB.
Fix some bugs that confused Overdef values with Undef values

Also implement improved caching for the PB analyser: create a temporary cachepoint at the loop preheader regardless of the outside caching situation, and clear it afterwards.

Finally make the store->load def-use graph construction more accurate: don't record clobbers, only record mustalias values. This actually misses partial-def dependencies but I haven't found a case where this matters yet.