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[MachineCopyPropagation] BackwardPropagatableCopy: add check for…

Authored by simonwallis2 on Jul 29 2020, 8:17 AM.

Description

[MachineCopyPropagation] BackwardPropagatableCopy: add check for hasOverlappingMultipleDef

In MachineCopyPropagation::BackwardPropagatableCopy(),
a check is added for multiple destination registers.

The copy propagation is avoided if the copied destination register
is the same register as another destination on the same instruction.

A new test is added. This used to fail on ARM like this:
error: unpredictable instruction, RdHi and RdLo must be different

umull   r9, r9, lr, r0

Reviewed By: lkail

Differential Revision: https://reviews.llvm.org/D82638