Hoist out some work done inside a loop doing a linear scan over all

Hoist out some work done inside a loop doing a linear scan over all
instructions in a block. GetUnderlyingObject is more expensive than it looks as
it can, for instance, call SimplifyInstruction.

This might have some behavioural changes in odd corner cases, but only because
of some strange artefacts of the original implementation. If you were relying
on those, we can fix that by replacing this with a smarter algorithm. Change
passes the existing tests.

llvm-svn: 166754