[AArch64][GlobalISel] Don't reconvert to p0 in convertPtrAddToAdd().

convertPtrAddToAdd improved overall code size and quality by a significant amount,
but on -O0 we generate some cross-class copies due to the fact that we emitted
G_PTRTOINT and G_INTTOPTR around the G_ADD. Unfortunately at -O0 we don't run any
register coalescing, so these cross class copies end up escaping as moves, and
we ended up regressing 3 benchmarks on CTMark (though still a winner overall).

This patch changes the lowering to instead directly emit the G_ADD into the
destination register, and then force changes the dest LLT to s64 from p0. This
should be ok, as all uses of the register should now be selected and therefore
the LLT doesn't matter for the users. It does however matter for the importer
patterns, which will fail to select a G_ADD if there's a p0 LLT.

I'm not able to get rid of the G_PTRTOINT on the source yet however. We can't
use the same trick of breaking the type system since that could break the
selection of the defining instruction. Thus with -O0 we still end up with a
cross class copy on source.

Code size improvements on -O0:
Program                                         baseline      new         diff
 test-suite :: CTMark/Bullet/bullet.test        965520       949164      -1.7%
 test-suite...TMark/7zip/7zip-benchmark.test    1069456      1052600     -1.6%
 test-suite...ark/tramp3d-v4/tramp3d-v4.test    1213692      1199804     -1.1%
 test-suite...:: CTMark/sqlite3/sqlite3.test    421680       419736      -0.5%
 test-suite...-typeset/consumer-typeset.test    837076       833380      -0.4%
 test-suite :: CTMark/lencod/lencod.test        799712       796976      -0.3%
 test-suite...:: CTMark/ClamAV/clamscan.test    688264       686132      -0.3%
 test-suite :: CTMark/kimwitu++/kc.test         1002344      999648      -0.3%
 test-suite...Mark/mafft/pairlocalalign.test    422296       421768      -0.1%
 test-suite :: CTMark/SPASS/SPASS.test          656792       656532      -0.0%
 Geomean difference                                                      -0.6%

Differential Revision: https://reviews.llvm.org/D73910