Constrain PowerPC autovectorization to fix bug 15041.

Certain vector operations don't vectorize well with the current
PowerPC implementation.  Element insert/extract performs poorly
without VSX support because Altivec requires going through memory.
SREM, UREM, and VSELECT all produce bad scalar code.

There's a lot of work to do for the cost model before
autovectorization will be tuned well, and this is not an attempt to
address the larger problem.

llvm-svn: 174660