[TwoAddressInstruction] Fix stale LiveVariables info in processStatepoint

D129213 improves verification of LiveVariables, and caused
CodeGen/X86/statepoint-cmp-sunk-past-statepoint.ll to fail with:
*** Bad machine code: LiveVariables: Block should not be in AliveBlocks ***
after Two-Address instruction pass.

Fix it by clearing AliveBlocks for a register which is no longer used.

Differential Revision: https://reviews.llvm.org/D136445