[X86][CGP] Reduce memcmp() expansion to 2 load pairs (PR33914)

D35067/rL308322 attempted to support up to 4 load pairs for memcmp inlining which resulted in regressions for some optimized libc memcmp implementations (PR33914).

Until we can match these more optimal cases, this patch reduces the memcmp expansion to a maximum of 2 load pairs (which matches what we do for -Os).

This patch should be considered for the 5.0.0 release branch as well

Differential Revision: https://reviews.llvm.org/D35830

llvm-svn: 308986