Don't form PPC CTR loops for over-sized exit counts

Although you can't generate this from C on PPC64, if you have a loop using a
64-bit counter on PPC32 then you can't form a CTR-based loop for it. This had
been cauing the PPCCTRLoops pass to assert.

Thanks to Joerg Sonnenberger for providing a test case!

llvm-svn: 185361