[X86] In X86DomainReassignment.cpp add enclosed registers to EnclosedEdges
The variable X86DomainReassignment::EnclosedEdges is used to store registers that have been enclosed in some closure, so those registers will be ignored when create new closures. But there is no registers has ever been put into this set, so a single register can be enclosed in multiple closures, it significantly increase compile time.
This patch adds a register into EnclosedEdges when it is enclosed into a closure.
Differential Revision: https://reviews.llvm.org/D58646