Create a README.txt for lib/Analysis, and add an entry.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@83001 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Analysis/README.txt b/lib/Analysis/README.txt
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+Analysis Opportunities:
+
+//===---------------------------------------------------------------------===//
+
+In test/Transforms/LoopStrengthReduce/quadradic-exit-value.ll, the
+ScalarEvolution expression for %r is this:
+
+  {1,+,3,+,2}<loop>
+
+Outside the loop, this could be evaluated simply as (%n * %n), however
+ScalarEvolution currently evaluates it as
+
+  (-2 + (2 * (trunc i65 (((zext i64 (-2 + %n) to i65) * (zext i64 (-1 + %n) to i65)) /u 2) to i64)) + (3 * %n))
+
+In addition to being much more complicated, it involves i65 arithmetic,
+which is very inefficient when expanded into code.
+
+//===---------------------------------------------------------------------===//