[InstCombine] fix fmul reassociation to avoid creating an extra fdiv

This was supposed to be an NFC refactoring that will eventually allow
eliminating the isFast() predicate, but there's a rare possibility
that we would pessimize the code as shown in the test case because
we failed to check 'hasOneUse()' properly. This version also removes
an inefficiency of the old code; we would look for: 
(X * C) * C1 --> X * (C * C1)
...but that pattern is always handled by 
SimplifyAssociativeOrCommutative().

llvm-svn: 327404