Merging r228979:

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r228979 | majnemer | 2015-02-12 15:26:26 -0800 (Thu, 12 Feb 2015) | 8 lines

X86: Don't crash if we can't decode the pshufb mask

Constant pool entries are uniqued by their contents regardless of their
type.  This means that a pshufb can have a shuffle mask which isn't a
simple array of bytes.

The code path which attempts to decode the mask didn't check for
failure, causing PR22559.
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_36@228983 91177308-0d34-0410-b5e6-96231b3b80d8