LiveIntervalAnalysis: Calculate liveness even if a superreg is reserved.

A register unit may be allocatable and non-reserved but some of the
register(tuples) built with it are reserved. We still need to calculate
liveness in this case.

Note to out of tree targets: If you start seeing machine verifier errors
with this commit, it probably means that you do not properly mark super
registers of reserved register as reserved. See for example r292836 or
r292870 for example on how to fix that.

rdar://29996737

Differential Revision: https://reviews.llvm.org/D28881

llvm-svn: 292871