[TwoAddressInstructionPass] Bugfix in handling of sunk instructions.

[TwoAddressInstructionPass]  Bugfix in handling of sunk instructions.

An instruction returned by TII->convertToThreeAddress() may contain a %noreg
(undef) operand, which is not expected by tryInstructionTransform(). So if
this MI is sunk to a lower point in MBB, it must be skipped when later
encountered.

A new set SunkInstrs is used for this purpose.

Note: there is no test supplied here, as this was triggered on SystemZ while
working on a review of instruction flags. A test case for this bugfix will be
included in the upcoming SystemZ commit.

Review: Quentin Colombet
https://reviews.llvm.org/D40711

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319646 91177308-0d34-0410-b5e6-96231b3b80d8