[CGP / PowerPC] use direct compares if there's only one load per block in memcmp() expansion

I'd like to enable CGP memcmp expansion for x86, but the output from CGP would regress the 
special cases (memcmp(x,y,N) != 0 for N=1,2,4,8,16,32 bytes) that we already handle.

I'm not sure if we'll actually be able to produce the optimal code given the block-at-a-time 
limitation in the DAG. We might have to just avoid those special-cases here in CGP. But 
regardless of that, I think this is a win for the more general cases.

http://rise4fun.com/Alive/cbQ

Differential Revision: https://reviews.llvm.org/D33963

llvm-svn: 304849