AMDGPU: Fix implementation of isCanonicalized

If denormals are enabled, denormals are canonical.
Also fix a few other issues. minnum/maxnum are supposed
to canonicalize. Temporarily improve workaround for the
instruction behavior change in gfx9.

Handle selects and fcopysign.

The tests were also largely broken, since they were
checking for a flush used on some targets after the
store of the result.

llvm-svn: 339061