[MemFunctions] Add microbenchmarks for memory functions.
Memory functions (memcmp, memcpy, ...) are typically recognized by the
compiler and expanded to specific asm patterns when the size is known at
This will help catch any regressions in expansions.
Right now we're only testing memcmp (see context in D60318).
Subscribers: mgorny, llvm-commits, gchatelet, spatel
Differential Revision: https://reviews.llvm.org/D64082