[LegalizeDAG] Handle NeedInvert when expanding BR_CC

This is a follow-up fix to commit 03c8d6a0.
Seems like we now end up with NeedInvert being set in the result
from LegalizeSetCCCondCode more often than in the past, so we
need to handle NeedInvert when expanding BR_CC.

Not sure how to deal with the "Tmp4.getNode()" case properly,
but current assumption is that that code path isn't impacted
by the changes in 03c8d6a0 so we can simply move
the old assert into the if-branch and only handle NeedInvert in the
else-branch.

I think that the test case added here, for PowerPC, might have
failed also before commit 03c8d6a0. But we started
to hit the assert more often downstream when having merged that
commit.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D94762