Only convert allocas to scalars if it is profitable. The profitability metric I

chose is having a non-memcpy/memset use and being larger than any native integer
type. Originally I chose having an access of a size smaller than the total size
of the alloca, but this caused some minor issues on the spirit benchmark where
SRoA runs again after some inlining.

This fixes <rdar://problem/8613163>.

llvm-svn: 127718