fix memcpy/memset/memmove lowering when optimizing for size

Fixing MinSize attribute handling was discussed in D11363. 
This is a prerequisite patch to doing that.

The handling of OptSize when lowering mem* functions was broken
on Darwin because it wants to ignore -Os for these cases, but the
existing logic also made it ignore -Oz (MinSize).

The Linux change demonstrates a widespread problem. The backend
doesn't usually recognize the MinSize attribute by itself; it
assumes that if the MinSize attribute exists, then the OptSize 
attribute must also exist. 

Fixing this more generally will be a follow-on patch or two.

Differential Revision: http://reviews.llvm.org/D11568

llvm-svn: 243693