ARM32: stop allocating a register for null checks on instance calls #68100
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For interface tail calls we currently end up allocating two internal
registers where one of them is forced into R12. LSRA currently does not
always handle the low amount of freedom, in particular with some
jitstressregs scenarios.
Since LR is always free going into a call we can just stop allocating a
register for the null check and use LR always to avoid the problem and
also give LSRA a little less work to do.
Fix #66563
cc @dotnet/jit-contrib