[InstCombine] Adjusted pow-exp tests for Windows [NFC]

Summary: https://bugs.llvm.org/show_bug.cgi?id=42740

Reviewers: efriedma, hans

Reviewed By: hans

Subscribers: spatel, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65220

llvm-svn: 366925