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[IfCvt] Don't use pristine register for counting liveins for predicated…

Authored by dmgreen on Jul 11 2021, 6:45 AM.

Description

[IfCvt] Don't use pristine register for counting liveins for predicated instructions.

The test case here hits machine verifier problems. There are volatile
long loads that the results of do not get used, loading into two dead
registers. IfCvt will predicate them and as it does will add implicit
uses of the predicating registers due to thinking they are live in. As
nothing has used the register, the machine verifier disagrees that they
are really live and we end up with a failure.

The registers come from Pristine regs that LivePhysRegs counts as live.
This patch adds a addLiveInsNoPristines method to be used instead in
IfCvt, so that only really live in regs need to be added as implicit
operands.

Differential Revision: https://reviews.llvm.org/D90965