[DebugInfo] Only handle DBG_VALUE in InlineSpiller.

The instructions using registers should be DBG_VALUE and normal
instructions. Use isDebugValue() to filter out DBG_VALUE and add
an assert to ensure there is no other kind of debug instructions
using the registers.

Differential Revision: https://reviews.llvm.org/D46739

Patch by Hsiangkai Wang.

llvm-svn: 332427