X86: Simplify X86ISD::Wrapper operand checks. NFCI.

We only ever create TargetConstantPool, TargetJumpTable, TargetExternalSymbol,
TargetGlobalAddress, TargetGlobalTLSAddress, MCSymbol and TargetBlockAddress
nodes as operands of X86ISD::Wrapper nodes, so we can remove one check and
invert the other.

Also update the documentation comment for X86ISD::Wrapper.

Differential Revision: https://reviews.llvm.org/D26731

llvm-svn: 287160