[InstCombine] avoid crash from deleting an instruction that still has uses (PR43723) (3rd try)

Re-try because earlier attempts were reverted due to use-after-free.
Hopefully, diagnosed correctly this time - we replace/remove the
invariant.start first rather than the invariant.end to avoid angering
worklist-based iteration.

We gather a set of white-listed instructions in isAllocSiteRemovable() and then
replace/erase them. But we don't know in general if the instructions in the set
have uses amongst themselves, so order of deletion makes a difference.

There's already a special-case for the llvm.objectsize intrinsic, so add another
for llvm.invariant.start.

Should fix:
https://bugs.llvm.org/show_bug.cgi?id=43723

Differential Revision: https://reviews.llvm.org/D69977