[IfConversion] Bugfix: Don't use undef flag while adding use operands.

IfConversion used to always add the undef flag when adding a use operand
on a newly predicated instruction. This would be an operand for the register
being conditionally redefined. Due to the undef flag, the liveness of this
register prior to the predicated instruction would get lost.

This patch changes this so that such use operands are added only when the
register is live, without the undef flag.

Reviewed by Quentin Colombet.
http://reviews.llvm.org/D209077

llvm-svn: 273545