[Reassociation] Place moved instructions after landing pads

Reassociation's NegateValue moved instructions to the beginning of
blocks (after PHIs) without checking for exception handling pads.
It's possible for reassociation to move something into an exception
handling 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
exception handling pads.

If the block we want to move into contains a catchswitch, we cannot
move into it.  In that case just create a new neg as if we had not
found an existing neg to move.

Differential Revision: https://reviews.llvm.org/D61089

llvm-svn: 360262