Also convert SUBREG_TO_REG to a KILL when relevant, like the other subreg

Also convert SUBREG_TO_REG to a KILL when relevant, like the other subreg
instructions.

This does not affect codegen much because SUBREG_TO_REG is only used by X86 and
X86 does not use the register scavenger, but it prevents verifier errors.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@106583 91177308-0d34-0410-b5e6-96231b3b80d8