[AArch64][GlobalISel] Add a simple cross-regclass copy optimization post-selection.

This does some trivial cross-regclass folding, where we can either do some extra
constraining to eliminate the copy or modify uses to use a smaller regclass.

There are minor code size improvements on average.

Program                                       size.__text
                                              before         after           diff
tramp3d-v4/tramp3d-v4                         366000.00      366012.00       0.0%
mafft/pairlocalalign                          248196.00      248188.00      -0.0%
7zip/7zip-benchmark                           568612.00      568592.00      -0.0%
kimwitu++/kc                                  434704.00      434676.00      -0.0%
Bullet/bullet                                 456128.00      456096.00      -0.0%
sqlite3/sqlite3                               284136.00      284100.00      -0.0%
ClamAV/clamscan                               381492.00      381396.00      -0.0%
SPASS/SPASS                                   412052.00      411944.00      -0.0%
lencod/lencod                                 428060.00      427912.00      -0.0%
consumer-typeset/consumer-typeset             413148.00      411116.00      -0.5%
                           Geomean difference                               -0.1%

Differential Revision: https://reviews.llvm.org/D136793