Debug info: Fix PR22296 by omitting the DW_AT_location if we lost the

physical register that is described in a DBG_VALUE.

In the testcase the DBG_VALUE describing "p5" becomes unavailable
because the register its address is in is clobbered and we (currently)
aren't smart enough to realize that the value is rematerialized immediately
after the DBG_VALUE and/or is actually a stack slot.

llvm-svn: 227056