[llvm] RegisterPressure: Remove dead defs correctly (PR #222627)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > A register unit is dead if any def covering it is dead, regardless of operand order.
> 
> Why? I do recall someone telling me that this original fix is incorrect; for example, for `dead $eax = MOV32r0 xxx, implicit-def $rax`. Has some spec been updated?

We have no MIR spec. The only way I see how to interpret that case is that the upper have of $rax is live, and the lower half is dead 

https://github.com/llvm/llvm-project/pull/222627