Document enable-inline-memcpy-ld-st flag (#565)

Adds documentation of the optional inline-memcpy-ld-st flag.

Author: nasherm

docs/optimization-flags.md

@@ -2,8 +2,14 @@ Additional optimization flags
 =============================
 
 ## Additional loop unroll in the LTO pipeline
-In some cases it is benefitial to perform an additional loop unroll pass so that extra information becomes available to later passes, e.g. SROA. 
+In some cases it is benefitial to perform an additional loop unroll pass so that extra information becomes available to later passes, e.g. SROA.
 Use cases where this could be beneficial - multiple (N>=4) nested loops.
 
-### Usage: 
+### Usage:
     -Wl,-plugin-opt=-extra-LTO-loop-unroll=true/false
+
+## Inline memcpy with LD/ST instructions
+In some cases inlining of memcpy instructions performs best when using LD/ST instructions.
+
+### Usage:
+    -mllvm -enable-inline-memcpy-ld-st