This change is to implement following rules under the condition C_A and/or C_R
 
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 C_A: reassociation is allowed
 C_R: reciprocal of a constant C is appropriate, which means 
    - 1/C is exact, or 
    - reciprocal is allowed and 1/C is neither a special value nor a denormal.
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 rule1:  (X/C1) / C2 => X / (C2*C1)  (if C_A)
                     => X * (1/(C2*C1))  (if C_A && C_R)
 rule 2:  X*C1 / C2 => X * (C1/C2)  if C_A
 rule 3: (X/Y)/Z = > X/(Y*Z)  (if C_A && at least one of Y and Z is symbolic value)
 rule 4: Z/(X/Y) = > (Z*Y)/X  (similar to rule3)

 rule 5: C1/(X*C2) => (C1/C2) / X (if C_A)
 rule 6: C1/(X/C2) => (C1*C2) / X (if C_A)
 rule 7: C1/(C2/X) => (C1/C2) * X (if C_A)


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172488 91177308-0d34-0410-b5e6-96231b3b80d8