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[PowerPC] Remove SeenUse check when optimizing conditional branch in

Description

[PowerPC] Remove SeenUse check when optimizing conditional branch in
PPCPreEmitPeephole pass.

PPCPreEmitPeephole will convert a BC to B when the conditional branch is
based on a constant CR by CRSET or CRUNSET. This is added in
https://reviews.llvm.org/rL343100.

When the conditional branch is known to be always taken, all branches will
be removed and a new unconditional branch will be inserted. However, when
SeenUse is false the original patch will not remove the branches, but still
insert the new unconditional branch, update the successors and create
inconsistent IR. Compiling the synthetic testcase included can show the
problem we run into.

The patch simply removes the SeenUse condition when adding branches into
InstrsToErase set.

Differential Revision: https://reviews.llvm.org/D56041