[AMDGPU] Fix v_swap_b32 formation on physical registers

As explained in the comments, matchSwap matches:

// mov t, x
// mov x, y
// mov y, t

and turns it into:

// mov t, x (t is potentially dead and move eliminated)
// v_swap_b32 x, y

On physical registers we don't have full use-def chains so the check
for T being live-out was not working properly with subregs/superregs.

Differential Revision: https://reviews.llvm.org/D101546