[RegAllocFast] Don't insert kill flags of super-register for partial kill

When reusing a register for a new definition, the fast register allocator
used to insert a kill flag at the previous last use of that register to
inform later passes that this register is free between the redef and the
last use. However, this may be wrong when subregisters are involved.
Indeed, a partially redef would have trigger a kill of the full super
register, potentially wrongly marking all the other subregisters as
free. Given we don't track which lanes are still live, we cannot set the
kill flag in such case.

Note: This bug has been latent for about 7 years (r104056).

llvmg.org/PR33677

llvm-svn: 307428