Merging r229352:

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r229352 | majnemer | 2015-02-15 20:02:09 -0800 (Sun, 15 Feb 2015) | 8 lines

IR: Properly return nullptr when getAggregateElement is out-of-bounds

We didn't properly handle the out-of-bounds case for
ConstantAggregateZero and UndefValue.  This would manifest as a crash
when the constant folder was asked to fold a load of a constant global
whose struct type has no operands.

This fixes PR22595.
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_36@229588 91177308-0d34-0410-b5e6-96231b3b80d8