Fix PR5563, an expensive checks failure when running on

Fix PR5563, an expensive checks failure when running on
tests/Transforms/InstCombine/shufflemask-undef.ll.  If
anyone cares, the use of 2*e here (and the equivalent
all over the place in instcombine) seems wrong, though
harmless: it should really be twice the length of the
input vector.  I think shufflevector used to require
that the mask have the same length as the input, but I
don't think that's true any more.  I don't care enough
about vectors to do anything about this...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@89456 91177308-0d34-0410-b5e6-96231b3b80d8