[InstSimplify] Missed optimization in math expression: log10(pow(10.0,x)) == x, log2...

[InstSimplify] Missed optimization in math expression: log10(pow(10.0,x)) == x, log2(pow(2.0,x)) == x

Summary: This patch enables folding following instructions under -ffast-math flag: log10(pow(10.0,x)) -> x, log2(pow(2.0,x)) -> x

Reviewers: hfinkel, spatel, efriedma, craig.topper, zvi, majnemer, lebedev.ri

Reviewed By: spatel, lebedev.ri

Subscribers: lebedev.ri, llvm-commits

Differential Revision: https://reviews.llvm.org/D41940

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@352981 91177308-0d34-0410-b5e6-96231b3b80d8