Extend live debug values down the dominator tree by following copies.

The llvm.dbg.value intrinsic refers to SSA values, not virtual registers, so we
should be able to extend the range of a value by tracking that value through
register copies. This greatly improves the debug value tracking for function
arguments that for some reason are copied to a second virtual register at the
end of the entry block.

We only extend the debug value range where its register is killed. All original
llvm.dbg.value locations are still respected.

Copies from physical registers are ignored. That should not be a problem since
the entry block already adds DBG_VALUE instructions for the virtual registers
holding the function arguments.

llvm-svn: 127912

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