VirtRegMap: Add undef flag when reading undefined subregisters.

While completely undefined registers are easy to catch and get their
<undef> flag early in ProcessImplicitDefs/RegisterCoalescer reading from
a partially defined register where just the subreg happens to be
undefined is harder to catch so we only add the undef flag in the
virtual register rewriting step.

No testcase as I cannot reproduce the problem on any of the in-tree targets at
the moment.

This fixes rdar://21387089

Differential Revision: http://reviews.llvm.org/D10470

llvm-svn: 239838