[X86] Remove some code from combineShuffle that seems largely unnecessary with...

[X86] Remove some code from combineShuffle that seems largely unnecessary with widening legalization.

The test case that changed is probably better served through
allowing combineTruncatedArithmetic to create narrow vectors. It
also appears InstCombine would have simplified this test case
to remove the zext and trunc anyway.

llvm-svn: 368522