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[MachineVerifier] Add TiedOpsRewritten flag to fix verify two-address error

Authored by ZhangKang on Jun 9 2020, 12:39 AM.

Description

[MachineVerifier] Add TiedOpsRewritten flag to fix verify two-address error

Summary:
Currently, MachineVerifier will attempt to verify that tied operands
satisfy register constraints as soon as the function is no longer in
SSA form. However, PHIElimination will take the function out of SSA
form while TwoAddressInstructionPass will actually rewrite tied operands
to match the constraints. PHIElimination runs first in the pipeline.
Therefore, whenever the MachineVerifier is run after PHIElimination,
it will encounter verification errors on any tied operands.

This patch adds a function property called TiedOpsRewritten that will be
set by TwoAddressInstructionPass and will control when the verifier checks
tied operands.

Reviewed By: nemanjai

Differential Revision: https://reviews.llvm.org/D80538