Replace subregister uses when processing tied operands

This was for some reason skipping operands that are subregisters
instead of keeping the same subregister index.

v_movreld_b32 expects src0 to be the subregister of the tied
super register use/def.

e.g.

v_movreld_b32 v0, v9, <imp-def, tied3> v[0:3], <imp-use, tied2> v[0:3]

was being replaced with

v[4:7] = copy v[0:3]
v_movreld_b32 v0, v9, <imp-def, tied3> v[4:7], <imp-use, tied2> v[4:7],

which really writes to v[0:3]

llvm-svn: 279804