[LegalizeTypes] Don't use PromoteTargetBoolean in WidenVecOp_SETCC.

Similar to r374970, but I don't have a test for this.

PromoteTargetBoolean is intended to be use for legalizing an
operand that needs to be promoted. It picks its type based on
the return from getSetccResultType and is intended to be used
when we have freedom to pick the new type. But the return type
we need for WidenVecOp_SETCC is completely determined by the
type of the input node.

llvm-svn: 374972