[InstCombine] peek through fdiv to find a squared sqrt

[InstCombine] peek through fdiv to find a squared sqrt

A more general canonicalization between fdiv and fmul would not
handle this case because that would have to be limited by uses
to prevent 2 values from becoming 3 values:
(x/y) * (x/y) --> (x*x) / (y*y)

(But we probably should still have that limited -- but more general --
canonicalization independently of this change.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@357943 91177308-0d34-0410-b5e6-96231b3b80d8