AMDGPU: Fix tracking subreg defs when folding through reg_sequence

We weren't fully respecting the type of a def of an immediate vs.
the type at the use point. Refactor the folding logic to track the
value to fold, as well as a subregister to apply to the underlying
value. This is similar to how PeepholeOpt tracks subregisters (though
only for pure copy-like instructions, no constants).

Fixes #139317

Author: arsenm