RegAllocGreedy: Fix subranges when rematerializing dead subreg defs

This would create a new interval missing the subrange and hit this
verifier error:

*** Bad machine code: Live interval for subreg operand has no subranges ***
- function:    test_remat_subreg_def
- basic block: %bb.0  (0xa568758) [0B;128B)
- instruction: 32B	dead undef %4.sub0:vreg_64 = V_MOV_B32_e32 2, implicit $exec