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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228979 91177308-0d34-0410-b5e6-96231b3b80d8