Fix Sub-register Rewriting in Aggressive Anti-Dependence Breaker

Previously, HandleLastUse would delete RegRef information for sub-registers
if they were dead even if their corresponding super-register were still live.

If the super-register were later renamed, then the definitions of the
sub-register would not be updated appropriately. This patch alters the
behavior so that RegInfo information for sub-registers is only deleted when
the sub-register and super-register are both dead.

This resolves PR26775. This is the mirror image of Hal's r227311 commit.

Author: Tom Jablin (tjablin)
Reviewers: kbarton uweigand nemanjai hfinkel

http://reviews.llvm.org/D18448

llvm-svn: 265097