This PR aims to reland work done by @arsenm which was previously
reverted due to some tangentially related scheduler issues as discussed
on #76416.
This PR cherry-picks the original commit (0e46b49de433), and adds
another patch on top with the following changes:
* The code in `updateRegDefsUses` now updates subranges when
subreg-liveness-tracking is enabled.
* When adding an implicit-def operand for the super-register,
the code in `reMaterializeTrivialDef` which tries to remove
undefined subranges should now take into account that the lanes
from the super-reg are no longer undefined.
Co-authored-by: Matt Arsenault <Matthew.Arsenault@amd.com>
4df6d3df24