[MachineCopyPropagation] More robust isForwardableRegClassCopy

Change the implementation of isForwardableRegClassCopy so that it
does not rely on getMinimalPhysRegClass. Instead, iterate over all
classes looking for any that satisfy a required property.

NFCI on current upstream targets, but this copes better with
downstream AMDGPU changes where some new smaller classes have been
introduced, which was breaking regclass equality tests in the old
code like:
    if (UseDstRC != CrossCopyRC && CopyDstRC == CrossCopyRC)

Differential Revision: https://reviews.llvm.org/D121903