[PATCH] Fold selects when the non-selected elements are undef from shufflevectors

[Filipe Cabecinhas]

Hi bkramer, eli.friedman,

If we have a select whose operands are two shufflevectors and we know the
non-selected elements are undef, replace the instruction with a
shufflevector from the source vectors of the shufflevectors.

Added test cases.

http://reviews.llvm.org/D3561

Files:
  lib/Transforms/InstCombine/InstCombineSelect.cpp
  test/Transforms/InstCombine/select.ll
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