[docs][PerformanceTips] Add text on allocas and alignment

This summarizes two recent llvm-dev discussions.  Most of the text provided by David Chisnall and Benoit Belley with minor editting by me.  



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247301 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed