The accumulator tail recursion transform claims to work for any associative

operation, but the way it's implemented requires the operation to also be
commutative.  So add a check for commutativity (and tweak the corresponding
comments).  This makes no difference in practice since every associative
LLVM instruction is also commutative!  Here's an example to show the need
for commutativity: the accum_recursion.ll testcase calculates the factorial
function.  Before the transformation the result of a call is
  ((((1*1)*2)*3)...)*x
while afterwards it is
  (((1*x)*(x-1))...*2)*1
which clearly requires both associativity and commutativity of * to be equal
to the original.

llvm-svn: 108056