Amara Emerson bbbe8ecc17 [GlobalISel][Localizer] Allow localization of a small number of repeated phi uses. (#77566) ...

We previously had a heuristic that if a value V was used multiple times
in a single PHI, then to avoid potentially rematerializing into many predecessors
we bail out. The phi uses only counted as a single use in the shouldLocalize() hook
because it counted the PHI as a single instruction use, not factoring in it may
have many incoming edges.

It turns out this heuristic is slightly too pessimistic, and allowing a small number
of these uses to be localized can improve code size due to shortening live ranges,
especially if those ranges span a call.

This change results in some improvements in size on CTMark -Os:
```
Program                                       size.__text
                                              before         after           diff
kimwitu++/kc                                  451676.00      451860.00       0.0%
mafft/pairlocalalign                          241460.00      241540.00       0.0%
tramp3d-v4/tramp3d-v4                         389216.00      389208.00      -0.0%
7zip/7zip-benchmark                           587528.00      587464.00      -0.0%
Bullet/bullet                                 457424.00      457348.00      -0.0%
consumer-typeset/consumer-typeset             405472.00      405376.00      -0.0%
SPASS/SPASS                                   410288.00      410120.00      -0.0%
lencod/lencod                                 426396.00      426108.00      -0.1%
ClamAV/clamscan                               380108.00      379756.00      -0.1%
sqlite3/sqlite3                               283664.00      283372.00      -0.1%
                           Geomean difference                               -0.0%
```
I experimented with different variations and thresholds. Using 3 instead
of 2 resulted in a further 0.1% improvement on ClamAV but also regressed
sqlite3 by the same %.

2024-01-11 18:57:37 +08:00

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