AMDGPU/GlobalISel: Do not fcanonicalize const splat padded with undef

Recognize constant splat padded with undef in isCanonicalized.
Fcanonicalize will be removed by RemoveFcanonicalize in post-legalizer
combiner. We will treat undef as value that will result in a splat
in clamp combine after regbankselect.

Differential Revision: https://reviews.llvm.org/D104408