[InstCombine] Preserve debug value when simplifying cast-of-select

InstCombine has a cast transform that matches a cast-of-select:

  Orig = cast (Src = select Cond TV FV)

And tries to replace it with a select which has the cast folded in:

  NewSel = select Cond (cast TV) (cast FV)

The combiner does RAUW(Orig, NewSel), so any debug values for Orig would
survive the transform. But debug values for Src would be lost.

This patch teaches InstCombine to replace all debug uses of Src with
NewSel (taking care of doing any necessary DIExpression rewriting).

Differential Revision: https://reviews.llvm.org/D49270

llvm-svn: 337310