llvm-reduce: Add reduction for instruction defs

Try to insert an implicit_def to replace the instruction's value,
replacing the original instruction's def with a dead register. If all
defs are delete the instruction entirely.

This is pretty similar to the instruction reduction, but leaves the
new defs in the same place as the original instruction. This could
possibly replace it. I'm not sure if we should directly delete the
instructions here, or leave dead ones behind.

This could also further work to replace physical register defs.