[InstCombine][NFCI] Update test cases in onehot_merge.ll

Use both one bit and signbit shifting to check for one bit merge.

Reviewers: lebedev.ri, spatel, efriedma, craig.topper

Reviewed By: lebedev.ri

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63903

llvm-svn: 364857