Reassociation's NegateValue moved instructions to the beginning of
blocks (after PHIs) without checking for landingpad instructions.
It's possible for reassociation to happen within a landingpad block so
we need to make sure we don't move things too early in the block.
This change advances the insertion point past any landingpad.
If the insertion point is then a catchswitch, don't try to reuse an
existing neg but just create a new one at BI, as if we hadn't found
an existing neg at all.