[DAGCombine] Optimize pow(X, 0.75) to sqrt(X) * sqrt(sqrt(X))

The sqrt case is faster and we already do this for the case where
the exponent is 0.25. This adds the 0.75 case which is also not
sensitive to signed zeros.

Patch by Whitney Tsang (Whitney)

Differential revision: https://reviews.llvm.org/D57434

llvm-svn: 353557