swift234

[IRGen] Remove an extra null comparison in 'exact' checked_cast_br.

rdar://22666588

This change removes a comparison and a branch on every virtual call. Before this
change we were generating code for comparing the metadata to figure out if the
incoming instance is an 'exact' cast, and then we checked if the result of the
cast was zero. This is unnecessary because we can simply reuse the result of the
exact metadata comparison. Moreover, we know that the metadata instance can't be
zero because we've emitted a load to that address that did not trap.

  %1 = getelementptr inbounds %C4main1X, %C4main1X* %0 ...
  %.metadata = load %swift.type*, %swift.type** %1  // Loading %0
  %2 = icmp eq %swift.type* %.metadata, bitcast (...)
  %3 = icmp ne %C4main1X* %0, null   ; <-----------  %0 can't be null.
  %4 = and i1 %3, %2
  br i1 %4, label %5, label %7

Swift SVN r31920