[InstCombine] Preserve debug values for eliminable casts

A cast from A to B is eliminable if its result is casted to C, and if
the pair of casts could just be expressed as a single cast. E.g here,
%c1 is eliminable:

  %c1 = zext i16 %A to i32
  %c2 = sext i32 %c1 to i64

InstCombine optimizes away eliminable casts. This patch teaches it to
insert a dbg.value intrinsic pointing to the final result, so that local
variables pointing to the eliminable result are preserved.

Differential Revision: https://reviews.llvm.org/D42566

llvm-svn: 323570