[InstCombine] Missed optimization in math expression: squashing sqrt functions

Summary: This patch enables folding under -ffast-math flag sqrt(a) * sqrt(b) -> sqrt(a*b)

Reviewers: hfinkel, spatel, davide

Reviewed By: spatel, davide

Subscribers: davide, llvm-commits

Differential Revision: https://reviews.llvm.org/D41322