[PowerPC] No CTR loop if the candidate exiting block is in a different loop

The CTR loops pass will insert the decrementing branch instruction in an exiting
block for the loop being transformed. However if that block is part of another
loop as well (whether a nested loop or with irreducible CFG), it is not valid
to use that exiting block. In fact, if the loop hass irreducible CFG, we don't
bother analyzing it and we just bail on the transformation. In practice, this
doesn't lead to a noticeable reduction in the number of loops transformed by
this pass.

Fixes https://bugs.llvm.org/show_bug.cgi?id=37229

Differential Revision: https://reviews.llvm.org/D46162

llvm-svn: 331410