Ensure all defs referring to a virtual register are marked dead by addRegisterDead().

Ensure all defs referring to a virtual register are marked dead by addRegisterDead().

There can be multiple defs for a single virtual register when they are defining
sub-registers.

The missing <dead> flag was stopping the inline spiller from eliminating dead
code after rematerialization.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@128888 91177308-0d34-0410-b5e6-96231b3b80d8