AMDGPU/SI: Fix a test in wqm.ll to always use s_cbranch_vcc*

Summary:
We need to use floating-point compares to ensure that s_cbranch_vcc*
instructions are always generated.  With integer compares, future
optimizations could cause s_cbranch_scc* to be generated instead.

Reviewers: arsenm, nhaehnle

Subscribers: llvm-commits, kzhuravl

Differential Revision: https://reviews.llvm.org/D23401

llvm-svn: 279148