Fix assert in X86 backend.

Fix assert in X86 backend.

When running combine on an extract_vector_elt, it wants to look through
a bitcast to check if the argument to the bitcast was itself an
extract_vector_elt with particular operands.

However, it called getOperand() on the argument to the bitcast *before*
checking that the opcode was EXTRACT_VECTOR_ELT, assert-failing if there
were zero operands for the actual opcode.

Fix, and add trivial test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249891 91177308-0d34-0410-b5e6-96231b3b80d8