[RegisterScavenger] Fix handling of predicated instructions

Summary:
The RegisterScavenger explicitly ignores <kill> flags on operands of
predicated instructions and therefore assumes that such registers remain
live. When it then scavenges such a register, it inserts a spill of this
(killed) register. This is invalid code and gets flagged up by the 
verifier.

Nowadays kill flags are set correctly on predicated instructions. This
patch makes the Scavenger respect them.

The bug has so far only been triggered by an internal pass, so I don't
have a test case unfortunately.

Fixes PR23119.

Reviewers: hfinkel, tobiasvk_caf

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9039

llvm-svn: 239439