[SimplifyLibCalls] powf(x, sitofp(n)) -> powi(x, n)

Summary:
Partially solves https://bugs.llvm.org/show_bug.cgi?id=42190



Reviewers: spatel, nikic, efriedma

Reviewed By: efriedma

Subscribers: efriedma, nikic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63038

llvm-svn: 364940